diff --git a/backend/app/api/user.py b/backend/app/api/user.py
index 45a32f9e..01fb1c5c 100644
--- a/backend/app/api/user.py
+++ b/backend/app/api/user.py
@@ -7,7 +7,7 @@ from uuid import UUID
 from fastapi import APIRouter, HTTPException, Depends, Header
 from pydantic import BaseModel
 from sqlalchemy.ext.asyncio import AsyncSession
-from sqlalchemy import select, delete
+from sqlalchemy import select, delete, func
 from datetime import datetime
 
 from backend.app.db import get_db, User, UserSettings, Report
@@ -203,19 +203,45 @@ async def create_report(
     user: User = Depends(get_current_user_required),
     db: AsyncSession = Depends(get_db)
 ):
-    """Save a new report"""
+    """Save a report (upsert: update if same ticker/date/market_type/language exists)"""
+    # Normalize language: treat None as "zh-TW" to avoid NULL matching issues
+    language = report_data.language or "zh-TW"
+
+    # Check for existing report with same key to prevent duplicates
+    existing_result = await db.execute(
+        select(Report)
+        .where(Report.user_id == user.id)
+        .where(Report.ticker == report_data.ticker)
+        .where(Report.analysis_date == report_data.analysis_date)
+        .where(Report.market_type == report_data.market_type)
+        .where(func.coalesce(Report.language, "zh-TW") == language)
+    )
+    existing = existing_result.scalar_one_or_none()
+
+    if existing:
+        # Update existing report instead of creating a duplicate
+        existing.result = report_data.result
+        existing.language = language
+        await db.commit()
+        return {
+            "success": True,
+            "report_id": str(existing.id),
+            "message": "Report updated successfully"
+        }
+
+    # No existing report found — create new
     report = Report(
         user_id=user.id,
         ticker=report_data.ticker,
         market_type=report_data.market_type,
         analysis_date=report_data.analysis_date,
         result=report_data.result,
-        language=report_data.language
+        language=language,
     )
     db.add(report)
     await db.commit()
     await db.refresh(report)
-    
+
     return {
         "success": True,
         "report_id": str(report.id),
@@ -254,6 +280,47 @@ async def get_report(
     )
 
 
+@router.delete("/reports/cleanup-duplicates")
+async def cleanup_duplicate_reports(
+    user: User = Depends(get_current_user_required),
+    db: AsyncSession = Depends(get_db)
+):
+    """Remove duplicate reports, keeping only the most recent one per (ticker, analysis_date, market_type, language)"""
+    # Fetch all user reports ordered newest first
+    result = await db.execute(
+        select(Report)
+        .where(Report.user_id == user.id)
+        .order_by(Report.created_at.desc())
+    )
+    all_reports = result.scalars().all()
+
+    seen: set = set()
+    ids_to_delete: list = []
+
+    for report in all_reports:
+        # Normalize language
+        lang = report.language or "zh-TW"
+        key = (report.ticker, report.analysis_date, report.market_type, lang)
+        if key in seen:
+            ids_to_delete.append(report.id)
+        else:
+            seen.add(key)
+
+    if ids_to_delete:
+        await db.execute(
+            delete(Report)
+            .where(Report.user_id == user.id)
+            .where(Report.id.in_(ids_to_delete))
+        )
+        await db.commit()
+
+    return {
+        "success": True,
+        "deleted": len(ids_to_delete),
+        "message": f"Cleaned up {len(ids_to_delete)} duplicate reports"
+    }
+
+
 @router.delete("/reports/{report_id}")
 async def delete_report(
     report_id: str,
diff --git a/frontend/app/history/page.tsx b/frontend/app/history/page.tsx
index 32b35580..e11465c9 100644
--- a/frontend/app/history/page.tsx
+++ b/frontend/app/history/page.tsx
@@ -50,6 +50,7 @@ import {
   deleteCloudReport,
   saveCloudReport,
   isCloudSyncEnabled,
+  cleanupDuplicateCloudReports,
 } from "@/lib/user-api";
 // import { LoginPrompt } from "@/components/auth/login-button";
 import { PendingTaskRecovery } from "@/components/PendingTaskRecovery";
@@ -365,23 +366,12 @@ const parseUTCDate = (dateStr: string): Date => {
 
 /**
  * Helper to generate a unique signature for deduplication.
- * This ensures reports for the same ticker on the same day are not squashed together.
+ * Uses only stable key fields: ticker + date + market_type + language.
+ * Language is normalized to "zh-TW" when null/undefined to match backend behavior.
  */
 const getReportSignature = (report: any): string => {
-  const baseKey = `${report.ticker}_${report.analysis_date}_${report.market_type || 'us'}`;
-  
-  let contentHash = "";
-  if (report.result) {
-    if (report.result.reports?.trader_investment_plan) {
-      const plan = report.result.reports.trader_investment_plan;
-      contentHash = `${plan.length}_${plan.slice(-30).replace(/[\s\n\r]+/g, '')}`;
-    } else {
-      contentHash = JSON.stringify(report.result).length.toString();
-    }
-  }
-  
-  const langKey = report.language || "unknown_lang";
-  return `${baseKey}_${langKey}_${contentHash}`;
+  const lang = report.language || "zh-TW";
+  return `${report.ticker}_${report.analysis_date}_${report.market_type || 'us'}_${lang}`;
 };
 
 export default function HistoryPage() {
@@ -449,7 +439,18 @@ export default function HistoryPage() {
     }
 
     try {
-      // First auto-clean local duplicates that might exist from older flawed versions
+      // First cleanup backend duplicates (existing duplicates from old versions)
+      try {
+        const cleanupResult = await cleanupDuplicateCloudReports();
+        if (cleanupResult && cleanupResult.deleted > 0) {
+          console.log(`☁️ Cleaned up ${cleanupResult.deleted} duplicate reports from cloud DB`);
+          cloudReportsPromiseRef.current = null; // Force refresh after cleanup
+        }
+      } catch (err) {
+        console.warn("Cloud duplicate cleanup failed:", err);
+      }
+
+      // Then auto-clean local duplicates that might exist from older flawed versions
       try {
         const allLocal = await getAllReports();
         const seenSignatures = new Set<string>();
@@ -787,12 +788,38 @@ export default function HistoryPage() {
     setDeleting(true);
     try {
       const cloudId = (reportToDelete as any).cloudId;
+      const targetLang = reportToDelete.language || detectReportLanguage(reportToDelete.result?.reports);
 
       // IMPORTANT: Delete from BOTH cloud AND local to prevent re-sync issues
-      // 1. If cloud ID exists, delete from cloud
-      if (cloudId) {
-        console.log("🗑️ Deleting from cloud:", cloudId);
-        await deleteCloudReport(cloudId);
+      // 1. Delete from cloud: delete the specific report AND any other duplicates with the same key
+      try {
+        const allCloudReports = await fetchCloudReportsCached(true);
+        const matchingCloudIds = allCloudReports
+          .filter((r) => {
+            const lang = r.language || "zh-TW";
+            return (
+              r.ticker === reportToDelete.ticker &&
+              r.analysis_date === reportToDelete.analysis_date &&
+              r.market_type === reportToDelete.market_type &&
+              lang === (targetLang || "zh-TW")
+            );
+          })
+          .map((r) => r.id);
+
+        if (matchingCloudIds.length > 0) {
+          console.log(`🗑️ Deleting ${matchingCloudIds.length} cloud report(s):`, matchingCloudIds);
+          await Promise.all(matchingCloudIds.map((id) => deleteCloudReport(id)));
+        } else if (cloudId) {
+          // Fallback: delete by cloudId if no match found by key
+          console.log("🗑️ Deleting from cloud by ID:", cloudId);
+          await deleteCloudReport(cloudId);
+        }
+      } catch (cloudErr) {
+        console.warn("Could not delete cloud copy:", cloudErr);
+        // Fallback to original cloudId delete
+        if (cloudId) {
+          await deleteCloudReport(cloudId);
+        }
       }
 
       // 2. Always try to delete from local IndexedDB as well
diff --git a/frontend/lib/user-api.ts b/frontend/lib/user-api.ts
index 6d187049..2ec25d97 100644
--- a/frontend/lib/user-api.ts
+++ b/frontend/lib/user-api.ts
@@ -176,6 +176,26 @@ export async function deleteCloudReport(reportId: string): Promise<boolean> {
   }
 }
 
+/**
+ * Remove duplicate reports from cloud (keeps newest per ticker/date/market/language)
+ */
+export async function cleanupDuplicateCloudReports(): Promise<{ deleted: number } | null> {
+  if (!isCloudSyncEnabled()) return null;
+
+  try {
+    const response = await fetch(`${API_BASE}/api/user/reports/cleanup-duplicates`, {
+      method: "DELETE",
+      headers: getAuthHeaders(),
+    });
+
+    if (!response.ok) return null;
+    return await response.json();
+  } catch (error) {
+    console.error("Failed to cleanup duplicate cloud reports:", error);
+    return null;
+  }
+}
+
 /**
  * Get a single report by ID from cloud
  */