better import csv error

internal/banking/import.go

@@ -36,6 +36,50 @@ func looseFingerprint(f domain.Facts) string {
 func sameBookedMoney(a, b domain.Facts) bool {
 	return a.AccountID == b.AccountID && a.BookingDate == b.BookingDate && a.Amount == b.Amount && a.Currency == b.Currency
 }
+func sourceLabel(source string) string {
+	switch source {
+	case "enablebanking":
+		return "bank-synced"
+	case "n26_csv":
+		return "CSV"
+	default:
+		return "source " + strconv.Quote(source)
+	}
+}
+func normalizedDetail(value string) string {
+	return strings.Join(strings.Fields(value), " ")
+}
+func conflictValue(value string) string {
+	if value == "" {
+		return "none"
+	}
+	const maxRunes = 120
+	runes := []rune(value)
+	if len(runes) > maxRunes {
+		value = string(runes[:maxRunes]) + "…"
+	}
+	return strconv.Quote(value)
+}
+func crossSourceDetails(left, right domain.Facts) string {
+	fields := []struct {
+		name, left, right string
+	}{
+		{"value date", left.ValueDate, right.ValueDate},
+		{"description", normalizedDetail(left.RawDescription), normalizedDetail(right.RawDescription)},
+		{"counterparty", strings.ToLower(normalizedDetail(left.Counterparty)), strings.ToLower(normalizedDetail(right.Counterparty))},
+		{"counterparty IBAN", left.CounterpartyIBAN, right.CounterpartyIBAN},
+	}
+	differences := make([]string, 0, len(fields))
+	for _, field := range fields {
+		if field.left != field.right {
+			differences = append(differences, fmt.Sprintf("%s (%s %s; %s %s)", field.name, sourceLabel(left.Source), conflictValue(field.left), sourceLabel(right.Source), conflictValue(field.right)))
+		}
+	}
+	if len(differences) == 0 {
+		return "unrecorded transaction details"
+	}
+	return strings.Join(differences, ", ")
+}
 
 // NormalizeAndDedupe returns new records without mutating the input. Stable bank
 // entry references, scoped by account, source and debit/credit direction, take
@@ -62,16 +106,18 @@ func NormalizeAndDedupe(data domain.Dataset, incoming []domain.Facts) ([]domain.
 	existing := map[string]map[string]int{}
 	existingAnonymous := map[string]int{}
 	existingIDs := map[string]domain.Facts{}
-	loose := map[string]map[string]map[string]bool{}
+	loose := map[string]map[string]map[string]domain.Facts{}
 	addLoose := func(f domain.Facts, fp string) {
 		k := looseFingerprint(f)
 		if loose[k] == nil {
-			loose[k] = map[string]map[string]bool{}
+			loose[k] = map[string]map[string]domain.Facts{}
 		}
 		if loose[k][f.Source] == nil {
-			loose[k][f.Source] = map[string]bool{}
+			loose[k][f.Source] = map[string]domain.Facts{}
+		}
+		if _, exists := loose[k][f.Source][fp]; !exists {
+			loose[k][f.Source][fp] = f
 		}
-		loose[k][f.Source][fp] = true
 	}
 	for _, t := range data.Transactions {
 		f, err := normalizeFacts(t.Facts, accounts)
@@ -127,9 +173,9 @@ func NormalizeAndDedupe(data domain.Dataset, incoming []domain.Facts) ([]domain.
 		for _, f := range g.facts {
 			for source, fps := range loose[looseFingerprint(f)] {
 				if source != g.source {
-					for fp := range fps {
+					for fp, other := range fps {
 						if fp != g.fp {
-							return nil, fmt.Errorf("uncertain cross-source match on account %s at %s; reconcile differing bank/CSV records before importing", f.AccountID, f.BookingDate)
+							return nil, fmt.Errorf("%s and %s transactions overlap on %s for %s %s but differ in %s. Finance Duck cannot tell whether they are one transaction or two; to prevent double counting, nothing was imported. Compare both records, then correct or remove the duplicate before retrying", sourceLabel(f.Source), sourceLabel(other.Source), f.BookingDate, f.Amount, f.Currency, crossSourceDetails(f, other))
 						}
 					}
 				}

internal/banking/import_test.go

@@ -241,8 +241,9 @@ func TestCrossSourceExactMatchAndUncertainty(t *testing.T) {
 	}
 	api.RawDescription = "Different bank text"
 	matched, err = NormalizeAndDedupe(d, []domain.Facts{api})
-	if err == nil || matched != nil {
-		t.Fatal("uncertain overlap was silently counted")
+	const wantConflict = "bank-synced and CSV transactions overlap on 2026-09-01 for -12.30 EUR but differ in description (bank-synced \"Different bank text\"; CSV \"Lunch\"). Finance Duck cannot tell whether they are one transaction or two; to prevent double counting, nothing was imported. Compare both records, then correct or remove the duplicate before retrying"
+	if err == nil || err.Error() != wantConflict || matched != nil {
+		t.Fatalf("unexpected cross-source conflict: %v %+v", err, matched)
 	}
 	api.RawDescription = csv.RawDescription
 	b := api