Add duplicate detection for receipt uploads

Implemented duplicate detection to warn users when uploading receipts
that may already exist in the system.

Changes:
- Extended ReceiptUploadResult to include DuplicateWarning list
- Added CheckForDuplicatesAsync method to ReceiptManager that checks:
  1. Identical file content (same SHA256 hash)
  2. Same file name and size (potential duplicate saved/edited)
- Updated upload flow to check for duplicates before saving
- Enhanced Receipts page to:
  - Display warning alert when duplicates are detected
  - Show details of each potential duplicate (receipt ID, filename,
    upload date, mapped transaction, and reason)
  - Provide quick link to view the duplicate receipt
  - Use TempData to persist warnings across redirect

Duplicate detection runs on every upload and provides actionable
information to help users avoid uploading the same receipt multiple
times. The check uses hash-based detection for exact duplicates and
name+size matching for potential duplicates.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

MoneyMap/Pages/Receipts.cshtml

@@ -18,6 +18,39 @@
     </div>
 }
 
+<!-- Duplicate Warnings -->
+@if (Model.DuplicateWarnings.Any())
+{
+    <div class="alert alert-warning alert-dismissible fade show" role="alert">
+        <h5 class="alert-heading">⚠️ Potential Duplicates Detected</h5>
+        <p>The following receipt(s) may be duplicates of the one you just uploaded:</p>
+        <ul class="mb-0">
+            @foreach (var warning in Model.DuplicateWarnings)
+            {
+                <li>
+                    <strong>Receipt #@warning.ReceiptId</strong> - @warning.FileName
+                    <br />
+                    <small class="text-muted">
+                        Uploaded: @warning.UploadedAtUtc.ToLocalTime().ToString("yyyy-MM-dd HH:mm") |
+                        Reason: @warning.Reason
+                        @if (warning.TransactionId.HasValue)
+                        {
+                            <span> | Mapped to: @warning.TransactionName (Transaction #@warning.TransactionId)</span>
+                        }
+                        else
+                        {
+                            <span> | Status: Unmapped</span>
+                        }
+                    </small>
+                    <br />
+                    <a asp-page="/ViewReceipt" asp-route-id="@warning.ReceiptId" class="btn btn-sm btn-outline-primary mt-1">View Receipt #@warning.ReceiptId</a>
+                </li>
+            }
+        </ul>
+        <button type="button" class="btn-close" data-bs-dismiss="alert" aria-label="Close"></button>
+    </div>
+}
+
 <!-- Upload Section -->
 <div class="card shadow-sm mb-3">
     <div class="card-header">

MoneyMap/Pages/Receipts.cshtml.cs

@@ -29,9 +29,20 @@ namespace MoneyMap.Pages
         [TempData]
         public bool IsSuccess { get; set; }
 
+        [TempData]
+        public string? DuplicateWarningsJson { get; set; }
+
+        public List<DuplicateWarning> DuplicateWarnings { get; set; } = new();
+
         public async Task OnGetAsync()
         {
             await LoadReceiptsAsync();
+
+            // Deserialize duplicate warnings if present
+            if (!string.IsNullOrWhiteSpace(DuplicateWarningsJson))
+            {
+                DuplicateWarnings = System.Text.Json.JsonSerializer.Deserialize<List<DuplicateWarning>>(DuplicateWarningsJson) ?? new();
+            }
         }
 
         public async Task<IActionResult> OnPostUploadAsync()
@@ -47,9 +58,18 @@ namespace MoneyMap.Pages
             var result = await _receiptManager.UploadUnmappedReceiptAsync(UploadFile);
 
             if (result.IsSuccess)
+            {
+                if (result.DuplicateWarnings.Any())
+                {
+                    Message = $"Receipt uploaded successfully, but {result.DuplicateWarnings.Count} potential duplicate(s) detected.";
+                    IsSuccess = true;
+                    DuplicateWarningsJson = System.Text.Json.JsonSerializer.Serialize(result.DuplicateWarnings);
+                }
+                else
                 {
                     Message = "Receipt uploaded successfully!";
                     IsSuccess = true;
+                }
                 return RedirectToPage();
             }
             else

MoneyMap/Services/ReceiptManager.cs

@@ -86,7 +86,7 @@ namespace MoneyMap.Services
                 fileHash = BitConverter.ToString(hashBytes).Replace("-", "").ToLowerInvariant();
             }
 
-            // Check for duplicate (same transaction + same hash, or same hash if unmapped)
+            // Check for exact duplicate (same transaction + same hash)
             if (transactionId.HasValue)
             {
                 var existingReceipt = await _db.Receipts
@@ -96,6 +96,9 @@ namespace MoneyMap.Services
                     return ReceiptUploadResult.Failure("This receipt has already been uploaded for this transaction.");
             }
 
+            // Check for potential duplicates (same hash, same name+size)
+            var duplicateWarnings = await CheckForDuplicatesAsync(fileHash, file.FileName, file.Length);
+
             // Generate unique filename
             var storedFileName = $"{transactionId?.ToString() ?? "unmapped"}_{Guid.NewGuid()}{extension}";
             var filePath = Path.Combine(receiptsBasePath, storedFileName);
@@ -125,7 +128,55 @@ namespace MoneyMap.Services
             _db.Receipts.Add(receipt);
             await _db.SaveChangesAsync();
 
-            return ReceiptUploadResult.Success(receipt);
+            return ReceiptUploadResult.Success(receipt, duplicateWarnings);
+        }
+
+        private async Task<List<DuplicateWarning>> CheckForDuplicatesAsync(string fileHash, string fileName, long fileSize)
+        {
+            var warnings = new List<DuplicateWarning>();
+
+            // Check for receipts with same hash
+            var hashMatches = await _db.Receipts
+                .Include(r => r.Transaction)
+                .Where(r => r.FileHashSha256 == fileHash)
+                .ToListAsync();
+
+            foreach (var match in hashMatches)
+            {
+                warnings.Add(new DuplicateWarning
+                {
+                    ReceiptId = match.Id,
+                    FileName = match.FileName,
+                    UploadedAtUtc = match.UploadedAtUtc,
+                    TransactionId = match.TransactionId,
+                    TransactionName = match.Transaction?.Name,
+                    Reason = "Identical file content (same hash)"
+                });
+            }
+
+            // Check for receipts with same name and size (but different hash - might be resaved/edited)
+            if (!warnings.Any())
+            {
+                var nameAndSizeMatches = await _db.Receipts
+                    .Include(r => r.Transaction)
+                    .Where(r => r.FileName == fileName && r.FileSizeBytes == fileSize)
+                    .ToListAsync();
+
+                foreach (var match in nameAndSizeMatches)
+                {
+                    warnings.Add(new DuplicateWarning
+                    {
+                        ReceiptId = match.Id,
+                        FileName = match.FileName,
+                        UploadedAtUtc = match.UploadedAtUtc,
+                        TransactionId = match.TransactionId,
+                        TransactionName = match.Transaction?.Name,
+                        Reason = "Same file name and size"
+                    });
+                }
+            }
+
+            return warnings;
         }
 
         public async Task<bool> MapReceiptToTransactionAsync(long receiptId, long transactionId)
@@ -227,11 +278,22 @@ namespace MoneyMap.Services
         public bool IsSuccess { get; init; }
         public Receipt? Receipt { get; init; }
         public string? ErrorMessage { get; init; }
+        public List<DuplicateWarning> DuplicateWarnings { get; init; } = new();
 
-        public static ReceiptUploadResult Success(Receipt receipt) =>
+        public static ReceiptUploadResult Success(Receipt receipt, List<DuplicateWarning>? warnings = null) =>
-            new() { IsSuccess = true, Receipt = receipt };
+            new() { IsSuccess = true, Receipt = receipt, DuplicateWarnings = warnings ?? new() };
 
         public static ReceiptUploadResult Failure(string error) =>
             new() { IsSuccess = false, ErrorMessage = error };
     }
+
+    public class DuplicateWarning
+    {
+        public long ReceiptId { get; set; }
+        public string FileName { get; set; } = "";
+        public DateTime UploadedAtUtc { get; set; }
+        public long? TransactionId { get; set; }
+        public string? TransactionName { get; set; }
+        public string Reason { get; set; } = "";
+    }
 }