feat: add duplicate transaction detection and display in transactions page, enhancing user experience with visual indicators for duplicates

app/api/banking/duplicates/ids/route.ts

@@ -0,0 +1,20 @@
+import { NextResponse } from "next/server";
+import { transactionService } from "@/services/transaction.service";
+import { requireAuth } from "@/lib/auth-utils";
+
+export async function GET() {
+  const authError = await requireAuth();
+  if (authError) return authError;
+
+  try {
+    const duplicateIds = await transactionService.getDuplicateIds();
+    return NextResponse.json({ duplicateIds: Array.from(duplicateIds) });
+  } catch (error) {
+    console.error("Error finding duplicate IDs:", error);
+    return NextResponse.json(
+      { error: "Failed to find duplicate IDs" },
+      { status: 500 },
+    );
+  }
+}
+

app/transactions/page.tsx

@@ -15,6 +15,7 @@ import {
   useBankingMetadata,
   useTransactions,
   getTransactionsQueryKey,
+  useDuplicateIds,
 } from "@/lib/hooks";
 import { updateCategory } from "@/lib/store-db";
 import { useQueryClient } from "@tanstack/react-query";
@@ -65,24 +66,25 @@ export default function TransactionsPage() {
   const [showReconciled, setShowReconciled] = useState<string>("all");
   const [period, setPeriod] = useState<Period>("all");
   const [customStartDate, setCustomStartDate] = useState<Date | undefined>(
-    undefined,
+    undefined
   );
   const [customEndDate, setCustomEndDate] = useState<Date | undefined>(
-    undefined,
+    undefined
   );
   const [isCustomDatePickerOpen, setIsCustomDatePickerOpen] = useState(false);
   const [sortField, setSortField] = useState<SortField>("date");
   const [sortOrder, setSortOrder] = useState<SortOrder>("desc");
   const [selectedTransactions, setSelectedTransactions] = useState<Set<string>>(
-    new Set(),
+    new Set()
   );
   const [ruleDialogOpen, setRuleDialogOpen] = useState(false);
   const [ruleTransaction, setRuleTransaction] = useState<Transaction | null>(
-    null,
+    null
   );
   const [updatingTransactionIds, setUpdatingTransactionIds] = useState<
     Set<string>
   >(new Set());
+  const [showDuplicates, setShowDuplicates] = useState(false);
 
   // Get start date based on period
   const startDate = useMemo(() => {
@@ -164,6 +166,9 @@ export default function TransactionsPage() {
     invalidate: invalidateTransactions,
   } = useTransactions(transactionParams, !!metadata);
 
+  // Fetch duplicate IDs
+  const { data: duplicateIds = new Set<string>() } = useDuplicateIds();
+
   // For filter comboboxes, we'll use empty arrays for now
   // They can be enhanced later with separate queries if needed
   const transactionsForAccountFilter: Transaction[] = [];
@@ -182,7 +187,7 @@ export default function TransactionsPage() {
     // Use transactions from current page to find similar ones
     const normalizedDesc = normalizeDescription(ruleTransaction.description);
     const similarTransactions = transactionsData.transactions.filter(
-      (t) => normalizeDescription(t.description) === normalizedDesc,
+      (t) => normalizeDescription(t.description) === normalizedDesc
     );
 
     if (similarTransactions.length === 0) return null;
@@ -193,7 +198,7 @@ export default function TransactionsPage() {
       transactions: similarTransactions,
       totalAmount: similarTransactions.reduce((sum, t) => sum + t.amount, 0),
       suggestedKeyword: suggestKeyword(
-        similarTransactions.map((t) => t.description),
+        similarTransactions.map((t) => t.description)
       ),
     };
   }, [ruleTransaction, transactionsData]);
@@ -209,7 +214,7 @@ export default function TransactionsPage() {
 
       // 1. Add keyword to category
       const category = metadata.categories.find(
-        (c: { id: string }) => c.id === ruleData.categoryId,
+        (c: { id: string }) => c.id === ruleData.categoryId
       );
       if (!category) {
         throw new Error("Category not found");
@@ -217,7 +222,7 @@ export default function TransactionsPage() {
 
       // Check if keyword already exists
       const keywordExists = category.keywords.some(
-        (k: string) => k.toLowerCase() === ruleData.keyword.toLowerCase(),
+        (k: string) => k.toLowerCase() === ruleData.keyword.toLowerCase()
       );
 
       if (!keywordExists) {
@@ -235,8 +240,8 @@ export default function TransactionsPage() {
               method: "PUT",
               headers: { "Content-Type": "application/json" },
               body: JSON.stringify({ id, categoryId: ruleData.categoryId }),
-            }),
+            })
-          ),
+          )
         );
       }
 
@@ -245,7 +250,7 @@ export default function TransactionsPage() {
       queryClient.invalidateQueries({ queryKey: ["banking-metadata"] });
       setRuleDialogOpen(false);
     },
-    [metadata, queryClient],
+    [metadata, queryClient]
   );
 
   const invalidateAll = useCallback(() => {
@@ -272,7 +277,7 @@ export default function TransactionsPage() {
     if (!transactionsData) return;
 
     const transaction = transactionsData.transactions.find(
-      (t) => t.id === transactionId,
+      (t) => t.id === transactionId
     );
     if (!transaction) return;
 
@@ -297,7 +302,7 @@ export default function TransactionsPage() {
     if (!transactionsData) return;
 
     const transaction = transactionsData.transactions.find(
-      (t) => t.id === transactionId,
+      (t) => t.id === transactionId
     );
     if (!transaction || transaction.isReconciled) return;
 
@@ -320,12 +325,12 @@ export default function TransactionsPage() {
 
   const setCategory = async (
     transactionId: string,
-    categoryId: string | null,
+    categoryId: string | null
   ) => {
     if (!transactionsData) return;
 
     const transaction = transactionsData.transactions.find(
-      (t) => t.id === transactionId,
+      (t) => t.id === transactionId
     );
     if (!transaction) return;
 
@@ -350,7 +355,7 @@ export default function TransactionsPage() {
         return {
           ...oldData,
           transactions: oldData.transactions.map((t) =>
-            t.id === transactionId ? { ...t, categoryId } : t,
+            t.id === transactionId ? { ...t, categoryId } : t
           ),
         };
       });
@@ -370,7 +375,7 @@ export default function TransactionsPage() {
     if (!transactionsData) return;
 
     const transactionsToUpdate = transactionsData.transactions.filter((t) =>
-      selectedTransactions.has(t.id),
+      selectedTransactions.has(t.id)
     );
 
     setSelectedTransactions(new Set());
@@ -382,8 +387,8 @@ export default function TransactionsPage() {
             method: "PUT",
             headers: { "Content-Type": "application/json" },
             body: JSON.stringify({ ...t, isReconciled: reconciled }),
-          }),
+          })
-        ),
+        )
       );
       invalidateTransactions();
     } catch (error) {
@@ -395,7 +400,7 @@ export default function TransactionsPage() {
     if (!transactionsData) return;
 
     const transactionsToUpdate = transactionsData.transactions.filter((t) =>
-      selectedTransactions.has(t.id),
+      selectedTransactions.has(t.id)
     );
 
     const transactionIds = transactionsToUpdate.map((t) => t.id);
@@ -413,8 +418,8 @@ export default function TransactionsPage() {
             method: "PUT",
             headers: { "Content-Type": "application/json" },
             body: JSON.stringify({ ...t, categoryId }),
-          }),
+          })
-        ),
+        )
       );
 
       // Mise à jour directe du cache après succès
@@ -424,7 +429,7 @@ export default function TransactionsPage() {
         return {
           ...oldData,
           transactions: oldData.transactions.map((t) =>
-            transactionIds.includes(t.id) ? { ...t, categoryId } : t,
+            transactionIds.includes(t.id) ? { ...t, categoryId } : t
           ),
         };
       });
@@ -446,7 +451,7 @@ export default function TransactionsPage() {
       setSelectedTransactions(new Set());
     } else {
       setSelectedTransactions(
-        new Set(transactionsData.transactions.map((t) => t.id)),
+        new Set(transactionsData.transactions.map((t) => t.id))
       );
     }
   };
@@ -488,7 +493,7 @@ export default function TransactionsPage() {
       return {
         ...oldData,
         transactions: oldData.transactions.filter(
-          (t) => t.id !== transactionId,
+          (t) => t.id !== transactionId
         ),
         total: oldData.total - 1,
       };
@@ -499,13 +504,13 @@ export default function TransactionsPage() {
         `/api/banking/transactions?id=${transactionId}`,
         {
           method: "DELETE",
-        },
+        }
       );
 
       if (!response.ok) {
         const errorData = await response.json().catch(() => ({}));
         throw new Error(
-          errorData.error || `Failed to delete transaction: ${response.status}`,
+          errorData.error || `Failed to delete transaction: ${response.status}`
         );
       }
 
@@ -593,6 +598,8 @@ export default function TransactionsPage() {
         }}
         isCustomDatePickerOpen={isCustomDatePickerOpen}
         onCustomDatePickerOpenChange={setIsCustomDatePickerOpen}
+        showDuplicates={showDuplicates}
+        onShowDuplicatesChange={setShowDuplicates}
         accounts={metadata.accounts}
         folders={metadata.folders}
         categories={metadata.categories}
@@ -631,6 +638,8 @@ export default function TransactionsPage() {
             formatCurrency={formatCurrency}
             formatDate={formatDate}
             updatingTransactionIds={updatingTransactionIds}
+            duplicateIds={duplicateIds}
+            highlightDuplicates={showDuplicates}
           />
 
           {/* Pagination controls */}

components/transactions/transaction-filters.tsx

@@ -28,10 +28,12 @@ import {
   SheetTitle,
   SheetTrigger,
 } from "@/components/ui/sheet";
-import { Search, X, Filter, Wallet, Calendar } from "lucide-react";
+import { Search, X, Filter, Wallet, Calendar, Copy } from "lucide-react";
 import { format } from "date-fns";
 import { fr } from "date-fns/locale";
 import { useIsMobile } from "@/hooks/use-mobile";
+import { Checkbox } from "@/components/ui/checkbox";
+import { Label } from "@/components/ui/label";
 import type { Account, Category, Folder, Transaction } from "@/lib/types";
 
 type Period = "1month" | "3months" | "6months" | "12months" | "custom" | "all";
@@ -53,6 +55,8 @@ interface TransactionFiltersProps {
   onCustomEndDateChange: (date: Date | undefined) => void;
   isCustomDatePickerOpen: boolean;
   onCustomDatePickerOpenChange: (open: boolean) => void;
+  showDuplicates: boolean;
+  onShowDuplicatesChange: (show: boolean) => void;
   accounts: Account[];
   folders: Folder[];
   categories: Category[];
@@ -77,6 +81,8 @@ export function TransactionFilters({
   onCustomEndDateChange,
   isCustomDatePickerOpen,
   onCustomDatePickerOpenChange,
+  showDuplicates,
+  onShowDuplicatesChange,
   accounts,
   folders,
   categories,
@@ -153,6 +159,23 @@ export function TransactionFilters({
           </SelectContent>
         </Select>
 
+        <div className="flex items-center gap-2 px-3 py-2 rounded-md border border-border bg-card">
+          <Checkbox
+            id="show-duplicates"
+            checked={showDuplicates}
+            onCheckedChange={(checked) =>
+              onShowDuplicatesChange(checked === true)
+            }
+          />
+          <Label
+            htmlFor="show-duplicates"
+            className="text-sm font-normal cursor-pointer flex items-center gap-2"
+          >
+            <Copy className="h-4 w-4 text-muted-foreground" />
+            Afficher les doublons
+          </Label>
+        </div>
+
         {period === "custom" && (
           <Popover
             open={isCustomDatePickerOpen}
@@ -256,7 +279,7 @@ export function TransactionFilters({
         onRemoveCategory={(id) => {
           const newCategories = selectedCategories.filter((c) => c !== id);
           onCategoriesChange(
-            newCategories.length > 0 ? newCategories : ["all"],
+            newCategories.length > 0 ? newCategories : ["all"]
           );
         }}
         onClearCategories={() => onCategoriesChange(["all"])}
@@ -282,7 +305,8 @@ export function TransactionFilters({
       (!selectedAccounts.includes("all") ? selectedAccounts.length : 0) +
       (!selectedCategories.includes("all") ? selectedCategories.length : 0) +
       (showReconciled !== "all" ? 1 : 0) +
-      (period !== "all" ? 1 : 0);
+      (period !== "all" ? 1 : 0) +
+      (showDuplicates ? 1 : 0);
 
     return (
       <>
@@ -367,7 +391,7 @@ function ActiveFilters({
 
   const selectedAccs = accounts.filter((a) => selectedAccounts.includes(a.id));
   const selectedCats = categories.filter((c) =>
-    selectedCategories.includes(c.id),
+    selectedCategories.includes(c.id)
   );
   const isUncategorized = selectedCategories.includes("uncategorized");
 

components/transactions/transaction-table.tsx

@@ -25,6 +25,7 @@ import {
   Wand2,
   Trash2,
   Loader2,
+  AlertTriangle,
 } from "lucide-react";
 import { DropdownMenuSeparator } from "@/components/ui/dropdown-menu";
 import { cn } from "@/lib/utils";
@@ -52,6 +53,8 @@ interface TransactionTableProps {
   formatCurrency: (amount: number) => string;
   formatDate: (dateStr: string) => string;
   updatingTransactionIds?: Set<string>;
+  duplicateIds?: Set<string>;
+  highlightDuplicates?: boolean;
 }
 
 const ROW_HEIGHT = 72; // Hauteur approximative d'une ligne
@@ -145,6 +148,8 @@ export function TransactionTable({
   formatCurrency,
   formatDate,
   updatingTransactionIds = new Set(),
+  duplicateIds = new Set(),
+  highlightDuplicates = false,
 }: TransactionTableProps) {
   const [focusedIndex, setFocusedIndex] = useState<number | null>(null);
   const parentRef = useRef<HTMLDivElement>(null);
@@ -164,7 +169,7 @@ export function TransactionTable({
       setFocusedIndex(index);
       onMarkReconciled(transactionId);
     },
-    [onMarkReconciled],
+    [onMarkReconciled]
   );
 
   const handleKeyDown = useCallback(
@@ -193,7 +198,7 @@ export function TransactionTable({
         }
       }
     },
-    [focusedIndex, transactions, onMarkReconciled, virtualizer],
+    [focusedIndex, transactions, onMarkReconciled, virtualizer]
   );
 
   useEffect(() => {
@@ -210,7 +215,7 @@ export function TransactionTable({
     (accountId: string) => {
       return accounts.find((a) => a.id === accountId);
     },
-    [accounts],
+    [accounts]
   );
 
   const getCategory = useCallback(
@@ -218,7 +223,7 @@ export function TransactionTable({
       if (!categoryId) return null;
       return categories.find((c) => c.id === categoryId);
     },
-    [categories],
+    [categories]
   );
 
   return (
@@ -247,6 +252,8 @@ export function TransactionTable({
                   const account = getAccount(transaction.accountId);
                   const _category = getCategory(transaction.categoryId);
                   const isFocused = focusedIndex === virtualRow.index;
+                  const isDuplicate =
+                    highlightDuplicates && duplicateIds.has(transaction.id);
 
                   return (
                     <div
@@ -259,6 +266,10 @@ export function TransactionTable({
                         left: 0,
                         width: "100%",
                         transform: `translateY(${virtualRow.start}px)`,
+                        ...(isDuplicate && {
+                          backgroundColor: "rgb(254 252 232)", // yellow-50
+                          borderLeft: "4px solid rgb(234 179 8)", // yellow-500
+                        }),
                       }}
                       onClick={() => {
                         // Désactiver le pointage au clic sur mobile
@@ -270,6 +281,7 @@ export function TransactionTable({
                         "p-4 space-y-3 hover:bg-muted/50 cursor-pointer border-b border-border",
                         transaction.isReconciled && "bg-emerald-500/5",
                         isFocused && "bg-primary/10 ring-1 ring-primary/30",
+                        isDuplicate && "shadow-sm"
                       )}
                     >
                       <div className="flex items-start justify-between gap-2">
@@ -282,9 +294,23 @@ export function TransactionTable({
                             onClick={(e) => e.stopPropagation()}
                           />
                           <div className="flex-1 min-w-0">
-                            <p className="font-medium text-xs md:text-sm truncate">
+                            <div className="flex items-center gap-2">
-                              {transaction.description}
+                              <p className="font-medium text-xs md:text-sm truncate">
-                            </p>
+                                {transaction.description}
+                              </p>
+                              {isDuplicate && (
+                                <Tooltip delayDuration={200}>
+                                  <TooltipTrigger asChild>
+                                    <AlertTriangle className="h-3 w-3 md:h-4 md:w-4 text-[var(--warning)] shrink-0" />
+                                  </TooltipTrigger>
+                                  <TooltipContent>
+                                    <p>
+                                      Transaction en double (même somme et date)
+                                    </p>
+                                  </TooltipContent>
+                                </Tooltip>
+                              )}
+                            </div>
                             {transaction.memo && (
                               <p className="text-[10px] md:text-xs text-muted-foreground truncate mt-1">
                                 {transaction.memo}
@@ -297,7 +323,7 @@ export function TransactionTable({
                             "font-semibold tabular-nums text-sm md:text-base shrink-0",
                             transaction.amount >= 0
                               ? "text-emerald-600"
-                              : "text-red-600",
+                              : "text-red-600"
                           )}
                         >
                           {transaction.amount >= 0 ? "+" : ""}
@@ -332,7 +358,7 @@ export function TransactionTable({
                             showBadge
                             align="start"
                             disabled={updatingTransactionIds.has(
-                              transaction.id,
+                              transaction.id
                             )}
                           />
                         </div>
@@ -365,7 +391,7 @@ export function TransactionTable({
                                 e.stopPropagation();
                                 if (
                                   confirm(
-                                    `Êtes-vous sûr de vouloir supprimer cette transaction ?\n\n${transaction.description}\n${formatCurrency(transaction.amount)}`,
+                                    `Êtes-vous sûr de vouloir supprimer cette transaction ?\n\n${transaction.description}\n${formatCurrency(transaction.amount)}`
                                   )
                                 ) {
                                   onDelete(transaction.id);
@@ -455,6 +481,8 @@ export function TransactionTable({
                   const transaction = transactions[virtualRow.index];
                   const account = getAccount(transaction.accountId);
                   const isFocused = focusedIndex === virtualRow.index;
+                  const isDuplicate =
+                    highlightDuplicates && duplicateIds.has(transaction.id);
 
                   return (
                     <div
@@ -467,6 +495,10 @@ export function TransactionTable({
                         left: 0,
                         width: "100%",
                         transform: `translateY(${virtualRow.start}px)`,
+                        ...(isDuplicate && {
+                          backgroundColor: "rgb(254 252 232)", // yellow-50
+                          borderLeft: "4px solid rgb(234 179 8)", // yellow-500
+                        }),
                       }}
                       onClick={() =>
                         handleRowClick(virtualRow.index, transaction.id)
@@ -475,6 +507,7 @@ export function TransactionTable({
                         "grid grid-cols-[auto_120px_2fr_150px_180px_140px_auto_auto] gap-0 border-b border-border hover:bg-muted/50 cursor-pointer",
                         transaction.isReconciled && "bg-emerald-500/5",
                         isFocused && "bg-primary/10 ring-1 ring-primary/30",
+                        isDuplicate && "shadow-sm"
                       )}
                     >
                       <div className="p-3">
@@ -492,9 +525,23 @@ export function TransactionTable({
                         className="p-3 min-w-0 overflow-hidden"
                         onClick={(e) => e.stopPropagation()}
                       >
-                        <p className="font-medium text-sm truncate">
+                        <div className="flex items-center gap-2">
-                          {transaction.description}
+                          <p className="font-medium text-sm truncate">
-                        </p>
+                            {transaction.description}
+                          </p>
+                          {isDuplicate && (
+                            <Tooltip delayDuration={200}>
+                              <TooltipTrigger asChild>
+                                <AlertTriangle className="h-4 w-4 text-[var(--warning)] shrink-0" />
+                              </TooltipTrigger>
+                              <TooltipContent>
+                                <p>
+                                  Transaction en double (même somme et date)
+                                </p>
+                              </TooltipContent>
+                            </Tooltip>
+                          )}
+                        </div>
                         {transaction.memo && (
                           <DescriptionWithTooltip
                             description={transaction.memo}
@@ -529,7 +576,7 @@ export function TransactionTable({
                           "p-3 text-right font-semibold tabular-nums",
                           transaction.amount >= 0
                             ? "text-emerald-600"
-                            : "text-red-600",
+                            : "text-red-600"
                         )}
                       >
                         {transaction.amount >= 0 ? "+" : ""}
@@ -596,7 +643,7 @@ export function TransactionTable({
                                 e.stopPropagation();
                                 if (
                                   confirm(
-                                    `Êtes-vous sûr de vouloir supprimer cette transaction ?\n\n${transaction.description}\n${formatCurrency(transaction.amount)}`,
+                                    `Êtes-vous sûr de vouloir supprimer cette transaction ?\n\n${transaction.description}\n${formatCurrency(transaction.amount)}`
                                   )
                                 ) {
                                   onDelete(transaction.id);

lib/hooks.ts

@@ -83,7 +83,7 @@ export function useLocalStorage<T>(key: string, initialValue: T) {
 
 // Helper function to serialize transaction params into a query key
 export function getTransactionsQueryKey(
-  params: TransactionsPaginatedParams = {},
+  params: TransactionsPaginatedParams = {}
 ): (string | number)[] {
   const key: (string | number)[] = ["transactions"];
   if (params.limit) key.push(`limit:${params.limit}`);
@@ -106,7 +106,7 @@ export function getTransactionsQueryKey(
 
 export function useTransactions(
   params: TransactionsPaginatedParams = {},
-  enabled = true,
+  enabled = true
 ) {
   const queryClient = useQueryClient();
 
@@ -134,7 +134,7 @@ export function useTransactions(
       if (params.isReconciled !== undefined && params.isReconciled !== "all") {
         searchParams.set(
           "isReconciled",
-          params.isReconciled === true ? "true" : "false",
+          params.isReconciled === true ? "true" : "false"
         );
       }
       if (params.sortField) searchParams.set("sortField", params.sortField);
@@ -205,3 +205,18 @@ export function useAccountsWithStats() {
     staleTime: 60 * 1000, // 1 minute
   });
 }
+
+export function useDuplicateIds() {
+  return useQuery({
+    queryKey: ["duplicate-ids"],
+    queryFn: async (): Promise<Set<string>> => {
+      const response = await fetch("/api/banking/duplicates/ids");
+      if (!response.ok) {
+        throw new Error("Failed to fetch duplicate IDs");
+      }
+      const data = await response.json();
+      return new Set(data.duplicateIds || []);
+    },
+    staleTime: 30 * 1000, // 30 seconds
+  });
+}

services/transaction.service.ts

@@ -41,14 +41,14 @@ export const transactionService = {
 
     // Create sets for fast lookup
     const existingFitIdSet = new Set(
-      existingByFitId.map((t) => `${t.accountId}-${t.fitId}`),
+      existingByFitId.map((t) => `${t.accountId}-${t.fitId}`)
     );
 
     // Create set for duplicates by amount + date + description
     const existingCriteriaSet = new Set(
       allExistingTransactions.map(
-        (t) => `${t.accountId}-${t.date}-${t.amount}-${t.description}`,
+        (t) => `${t.accountId}-${t.date}-${t.amount}-${t.description}`
-      ),
+      )
     );
 
     // Filter out duplicates based on fitId OR (amount + date + description)
@@ -85,7 +85,7 @@ export const transactionService = {
 
   async update(
     id: string,
-    data: Partial<Omit<Transaction, "id">>,
+    data: Partial<Omit<Transaction, "id">>
   ): Promise<Transaction> {
     const updated = await prisma.transaction.update({
       where: { id },
@@ -123,14 +123,67 @@ export const transactionService = {
       await prisma.transaction.delete({
         where: { id },
       });
-    } catch (error: any) {
+    } catch (error: unknown) {
-      if (error.code === "P2025") {
+      if (
+        error &&
+        typeof error === "object" &&
+        "code" in error &&
+        error.code === "P2025"
+      ) {
         throw new Error(`Transaction with id ${id} not found`);
       }
       throw error;
     }
   },
 
+  async getDuplicateIds(): Promise<Set<string>> {
+    // Get all transactions grouped by account
+    const allTransactions = await prisma.transaction.findMany({
+      orderBy: [
+        { accountId: "asc" },
+        { date: "asc" },
+        { createdAt: "asc" }, // Oldest first
+      ],
+      select: {
+        id: true,
+        accountId: true,
+        date: true,
+        amount: true,
+      },
+    });
+
+    // Group by account for efficient processing
+    const transactionsByAccount = new Map<string, typeof allTransactions>();
+    for (const transaction of allTransactions) {
+      if (!transactionsByAccount.has(transaction.accountId)) {
+        transactionsByAccount.set(transaction.accountId, []);
+      }
+      transactionsByAccount.get(transaction.accountId)!.push(transaction);
+    }
+
+    const duplicateIds = new Set<string>();
+    const seenKeys = new Map<string, string>(); // key -> first transaction ID
+
+    // For each account, find duplicates by amount + date only
+    for (const [accountId, transactions] of transactionsByAccount.entries()) {
+      for (const transaction of transactions) {
+        const key = `${accountId}-${transaction.date}-${transaction.amount}`;
+
+        if (seenKeys.has(key)) {
+          // This is a duplicate - mark both the first and this one
+          const firstId = seenKeys.get(key)!;
+          duplicateIds.add(firstId);
+          duplicateIds.add(transaction.id);
+        } else {
+          // First occurrence
+          seenKeys.set(key, transaction.id);
+        }
+      }
+    }
+
+    return duplicateIds;
+  },
+
   async deduplicate(): Promise<{
     deletedCount: number;
     duplicatesFound: number;