Distinguish debit and credit postings sharing an upstream reference

internal/banking/import.go

@@ -18,7 +18,15 @@ func digest(parts ...string) string {
 	h := sha256.Sum256(b)
 	return hex.EncodeToString(h[:])
 }
-func identity(f domain.Facts) string { return digest(f.AccountID, f.Source, f.ExternalID) }
+func identity(f domain.Facts) string {
+	// Banks can reuse a reference for a debit and its credit counterpart.
+	// Facts are normalized before lookup; never rewrite stored journal IDs.
+	direction := "credit"
+	if strings.HasPrefix(string(f.Amount), "-") {
+		direction = "debit"
+	}
+	return digest(f.AccountID, f.Source, f.ExternalID, direction)
+}
 func fingerprint(f domain.Facts) string {
 	return digest(f.AccountID, f.BookingDate, f.ValueDate, f.Amount.String(), f.Currency, strings.Join(strings.Fields(f.RawDescription), " "), strings.ToLower(strings.Join(strings.Fields(f.Counterparty), " ")), f.CounterpartyIBAN)
 }
@@ -30,9 +38,10 @@ func sameBookedMoney(a, b domain.Facts) bool {
 }
 
 // NormalizeAndDedupe returns new records without mutating the input. Stable bank
-// entry references take precedence over text. CSV rows without references use
-// occurrence counts, not a set: two identical rows remain two transactions and
-// importing the same export again creates none. For overlapping partial exports,
+// entry references, scoped by account, source and debit/credit direction, take
+// precedence over text. Rows without references use occurrence counts, not a set:
+// two identical rows remain two transactions; reimporting creates none.
+// For overlapping partial exports,
 // indistinguishable rows cannot prove an additional occurrence; import complete
 // overlapping date windows to establish multiplicity.
 //

internal/banking/import_test.go

@@ -105,6 +105,104 @@ func TestUpstreamIdentityPreferredAndAccountScoped(t *testing.T) {
 		t.Fatal("changed immutable upstream money accepted")
 	}
 }
+func TestUpstreamReferenceSeparatesDebitAndCredit(t *testing.T) {
+	d := fixtureDataset()
+	debit := fixtureFacts()
+	debit.Source = "enablebanking"
+	debit.ExternalID = "shared-bank-reference"
+	credit := debit
+	credit.Amount = "12.30"
+	rows, err := NormalizeAndDedupe(d, []domain.Facts{debit, credit, debit, credit})
+	if err != nil {
+		t.Fatal(err)
+	}
+	amounts := map[domain.Money]int{}
+	for _, row := range rows {
+		amounts[row.Facts.Amount]++
+	}
+	if !reflect.DeepEqual(amounts, map[domain.Money]int{"-12.30": 1, "12.30": 1}) {
+		t.Fatalf("debit/credit postings lost or duplicated: %+v", rows)
+	}
+	d.Transactions = rows
+	if err := domain.Validate(d); err != nil {
+		t.Fatal(err)
+	}
+	again, err := NormalizeAndDedupe(d, []domain.Facts{credit, debit})
+	if err != nil || len(again) != 0 {
+		t.Fatalf("repeated debit/credit pair was not idempotent: %+v %v", again, err)
+	}
+}
+
+func TestUpstreamDirectionPreservesPreviouslyStoredIdentity(t *testing.T) {
+	for _, amount := range []domain.Money{"-12.30", "12.30"} {
+		t.Run(string(amount), func(t *testing.T) {
+			d := fixtureDataset()
+			stored := fixtureFacts()
+			stored.Source = "enablebanking"
+			stored.ExternalID = "shared-bank-reference"
+			stored.Amount = amount
+			// Reproduce the journal identity written before direction scoping.
+			stored.ID = "tx_" + digest(stored.AccountID, stored.Source, stored.ExternalID)
+			stored.Fingerprint = fingerprint(stored)
+			d.Transactions = []domain.Transaction{{Facts: stored, Enrichment: domain.Fallback(stored)}}
+			opposite := stored
+			opposite.Amount = "12.30"
+			if amount == "12.30" {
+				opposite.Amount = "-12.30"
+			}
+			incoming := stored
+			incoming.RawDescription = "Updated upstream display"
+			added, err := NormalizeAndDedupe(d, []domain.Facts{opposite, incoming})
+			if err != nil || len(added) != 1 || added[0].Facts.Amount != opposite.Amount {
+				t.Fatalf("stored posting duplicated or opposite posting lost: %+v %v", added, err)
+			}
+			if !reflect.DeepEqual(d.Transactions[0].Facts, stored) {
+				t.Fatal("previously stored bank facts were rewritten")
+			}
+			d.Transactions = append(d.Transactions, added...)
+			if err := domain.Validate(d); err != nil {
+				t.Fatal(err)
+			}
+			again, err := NormalizeAndDedupe(d, []domain.Facts{incoming, opposite})
+			if err != nil || len(again) != 0 {
+				t.Fatalf("upgraded journal was not idempotent: %+v %v", again, err)
+			}
+		})
+	}
+}
+
+func TestUpstreamReferenceRejectsSameDirectionConflicts(t *testing.T) {
+	original := fixtureFacts()
+	original.Source = "enablebanking"
+	original.ExternalID = "shared-bank-reference"
+	original.Amount = "12.30"
+	for _, tc := range []struct {
+		name   string
+		change func(*domain.Facts)
+	}{
+		{"amount", func(f *domain.Facts) { f.Amount = "99.00" }},
+		{"booking date", func(f *domain.Facts) { f.BookingDate = "2026-09-02" }},
+		{"currency", func(f *domain.Facts) { f.Currency = "USD" }},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			d := fixtureDataset()
+			changed := original
+			tc.change(&changed)
+			if added, err := NormalizeAndDedupe(d, []domain.Facts{original, changed}); err == nil || added != nil {
+				t.Fatal("conflicting incoming postings were accepted")
+			}
+			var err error
+			d.Transactions, err = NormalizeAndDedupe(d, []domain.Facts{original})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if added, err := NormalizeAndDedupe(d, []domain.Facts{changed}); err == nil || added != nil {
+				t.Fatal("changed stored booking facts were accepted")
+			}
+		})
+	}
+}
+
 func TestDistinctUpstreamIDsPreserveEqualTransactions(t *testing.T) {
 	d := fixtureDataset()
 	a := fixtureFacts()