feat: duplicate detection on CSV import

Compute a sha256 fingerprint (date|description|amount|account_id, first
16 hex chars) for every CSV row and store it as bank_ref. At preview
time, existing fingerprints are fetched and matching rows are shown
greyed out with a "duplicate" label. At confirm time, those rows are
silently skipped — only truly new transactions are inserted.

If every row is a duplicate the user is redirected with ?notice=all_duplicates
instead of inserting an empty batch.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/finance/services/api/main/handler.go

@@ -2,7 +2,9 @@ package main
 
 import (
 	"context"
+	"crypto/sha256"
 	"embed"
+	"encoding/hex"
 	"encoding/json"
 	"fmt"
 	"html/template"
@@ -792,8 +794,24 @@ func (h *Handler) ImportPreview(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	// compute fingerprints and detect duplicates
+	var fingerprints []string
 	for i := range rows {
 		rows[i].Category = autoCategorize(rows[i].Description, catMap)
+		rows[i].Fingerprint = txnFingerprint(rows[i].Date, rows[i].Description, rows[i].AmountCents, accountID)
+		fingerprints = append(fingerprints, rows[i].Fingerprint)
+	}
+	existing, _ := h.store.getTransactions(ctx, a.UserID, bson.M{"bank_ref": bson.M{"$in": fingerprints}})
+	existingRefs := map[string]bool{}
+	for _, t := range existing {
+		existingRefs[t.BankRef] = true
+	}
+	duplicateCount := 0
+	for i := range rows {
+		if existingRefs[rows[i].Fingerprint] {
+			rows[i].Duplicate = true
+			duplicateCount++
+		}
 	}
 
 	importPreview := &CSVImportPreview{
@@ -815,6 +833,7 @@ func (h *Handler) ImportPreview(w http.ResponseWriter, r *http.Request) {
 		"SelectedFormat":  string(format),
 		"SelectedAccount": accountID,
 		"CategoryColors":  catColors,
+		"DuplicateCount":  duplicateCount,
 	})
 }
 
@@ -861,9 +880,24 @@ func (h *Handler) ImportConfirm(w http.ResponseWriter, r *http.Request) {
 
 	userCats := r.Form["categories"]
 
+	// compute fingerprints and skip duplicates
+	var fingerprints []string
+	for _, row := range rows {
+		fingerprints = append(fingerprints, txnFingerprint(row.Date, row.Description, row.AmountCents, accountID))
+	}
+	existing, _ := h.store.getTransactions(ctx, a.UserID, bson.M{"bank_ref": bson.M{"$in": fingerprints}})
+	existingRefs := map[string]bool{}
+	for _, t := range existing {
+		existingRefs[t.BankRef] = true
+	}
+
 	now := time.Now()
 	var txns []Transaction
 	for i, row := range rows {
+		fp := fingerprints[i]
+		if existingRefs[fp] {
+			continue
+		}
 		date, _ := time.Parse("2006-01-02", row.Date)
 		cat := "Others"
 		if i < len(userCats) && userCats[i] != "" {
@@ -878,10 +912,16 @@ func (h *Handler) ImportConfirm(w http.ResponseWriter, r *http.Request) {
 			Description: row.Description,
 			AmountCents: row.AmountCents,
 			Category:    cat,
+			BankRef:     fp,
 			CreatedAt:   now,
 		})
 	}
 
+	if len(txns) == 0 {
+		http.Redirect(w, r, "/transactions?notice=all_duplicates", http.StatusSeeOther)
+		return
+	}
+
 	if err := h.store.createTransactions(ctx, txns); err != nil {
 		slog.Error("create transactions", "err", err)
 		http.Error(w, "save error", http.StatusInternalServerError)
@@ -891,6 +931,11 @@ func (h *Handler) ImportConfirm(w http.ResponseWriter, r *http.Request) {
 	http.Redirect(w, r, "/transactions", http.StatusSeeOther)
 }
 
+func txnFingerprint(date, description string, amountCents int64, accountID string) string {
+	h := sha256.Sum256([]byte(fmt.Sprintf("%s|%s|%d|%s", date, description, amountCents, accountID)))
+	return hex.EncodeToString(h[:])[:16]
+}
+
 func autoCategorize(desc string, catMap map[string]string) string {
 	desc = strings.ToLower(desc)
 	keywords := map[string]string{

apps/finance/services/api/main/models.go

@@ -96,6 +96,8 @@ type CSVImportRow struct {
 	Description string `json:"description"`
 	AmountCents int64  `json:"amount_cents"`
 	Category    string `json:"category"`
+	Fingerprint string `json:"fingerprint"`
+	Duplicate   bool   `json:"duplicate"`
 }
 
 type CSVImportPreview struct {

apps/finance/services/api/main/templates/import.html

@@ -14,7 +14,12 @@
   <div style="display:flex; justify-content:space-between; align-items:center; margin-bottom:16px; flex-wrap:wrap; gap:8px;">
     <div>
       <h2 style="font-size:15px; font-weight:700; color:var(--text); text-transform:none; letter-spacing:0;">Preview</h2>
-      <p class="text-muted" style="margin-top:3px;">{{$d.Preview.Total}} rows — review categories before confirming.</p>
+      <p class="text-muted" style="margin-top:3px;">
+        {{$d.Preview.Total}} rows
+        {{if $d.DuplicateCount}}
+        — <span style="color:var(--yellow, #f59e0b); font-weight:600;">{{$d.DuplicateCount}} already imported</span> (shown greyed out, will be skipped)
+        {{end}}
+      </p>
     </div>
     <a href="/import" class="btn btn-outline btn-sm">← Back</a>
   </div>
@@ -36,13 +41,19 @@
         </thead>
         <tbody>
           {{range $i, $row := $d.Preview.Rows}}
-          <tr>
+          <tr {{if $row.Duplicate}}style="opacity:0.4;"{{end}}>
             <td style="white-space:nowrap; color:var(--text2);">{{$row.Date}}</td>
-            <td style="max-width:280px; overflow:hidden; text-overflow:ellipsis; white-space:nowrap;">{{$row.Description}}</td>
+            <td style="max-width:280px; overflow:hidden; text-overflow:ellipsis; white-space:nowrap;">
+              {{$row.Description}}
+              {{if $row.Duplicate}}<span style="font-size:11px; font-weight:600; color:var(--muted); margin-left:6px;">duplicate</span>{{end}}
+            </td>
             <td class="cents {{if lt $row.AmountCents 0}}negative{{else}}positive{{end}}" style="font-weight:600; white-space:nowrap;">
               {{if lt $row.AmountCents 0}}−{{else}}+{{end}}€{{cents (centsAbs $row.AmountCents)}}
             </td>
             <td>
+              {{if $row.Duplicate}}
+              <span style="font-size:12px; color:var(--muted);">—</span>
+              {{else}}
               <select name="categories" class="cat-select" data-selected="{{$row.Category}}"
                       style="font-size:12.5px; padding:5px 8px; border:1.5px solid var(--border2);
                              border-radius:6px; background:var(--bg2); color:var(--text);
@@ -51,6 +62,7 @@
                 <option value="{{$cat}}" {{if eq $cat $row.Category}}selected{{end}}>{{$cat}}</option>
                 {{end}}
               </select>
+              {{end}}
             </td>
           </tr>
           {{end}}