diff --git a/backend/infra/migrations/010_add_unique_email_index.sql b/backend/infra/migrations/010_add_unique_email_index.sql
new file mode 100644
index 0000000..a51149e
--- /dev/null
+++ b/backend/infra/migrations/010_add_unique_email_index.sql
@@ -0,0 +1,20 @@
+-- Migration 010: enforce unique emails in users
+-- Matches runtime expectation for idx_users_email_unique.
+
+BEGIN;
+
+DO $$
+BEGIN
+  IF EXISTS (
+    SELECT 1
+    FROM users
+    GROUP BY email
+    HAVING COUNT(*) > 1
+  ) THEN
+    RAISE EXCEPTION 'Cannot create unique index idx_users_email_unique: duplicate emails exist in users';
+  END IF;
+END $$;
+
+CREATE UNIQUE INDEX IF NOT EXISTS idx_users_email_unique ON users(email);
+
+COMMIT;
diff --git a/backend/internal/models/user.go b/backend/internal/models/user.go
index 1481c68..9f5b4af 100644
--- a/backend/internal/models/user.go
+++ b/backend/internal/models/user.go
@@ -5,6 +5,7 @@ import (
 	"errors"
 
 	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/jackc/pgx/v5/pgxpool"
 	"github.com/kjannette/koin-ping/backend/internal/domain"
 )
@@ -37,10 +38,55 @@ func scanUser(row pgx.Row) (*domain.User, error) {
 	return &u, nil
 }
 
+func (m *UserModel) getByFirebaseUID(ctx context.Context, firebaseUID string) (*domain.User, error) {
+	row := m.pool.QueryRow(ctx,
+		`SELECT `+userColumns+` FROM users WHERE firebase_uid = $1`,
+		firebaseUID,
+	)
+	return scanUser(row)
+}
+
+func (m *UserModel) getByEmail(ctx context.Context, email string) (*domain.User, error) {
+	row := m.pool.QueryRow(ctx,
+		`SELECT `+userColumns+` FROM users WHERE email = $1`,
+		email,
+	)
+	return scanUser(row)
+}
+
+func isUniqueViolation(err error) bool {
+	var pgErr *pgconn.PgError
+	if errors.As(err, &pgErr) {
+		return pgErr.Code == "23505"
+	}
+	return false
+}
+
 // FindOrCreateByFirebaseUID returns the local user for a Firebase UID,
-// creating one if it doesn't exist yet. On conflict (returning user) the
-// updated_at timestamp is refreshed.
+// creating one if it doesn't exist yet.
+//
+// For onboarding races or legacy duplicate-identity states, this method
+// gracefully falls back to an existing row by email instead of failing
+// with a unique-email violation.
 func (m *UserModel) FindOrCreateByFirebaseUID(ctx context.Context, firebaseUID, email string) (*domain.User, error) {
+	existingByUID, err := m.getByFirebaseUID(ctx, firebaseUID)
+	if err != nil {
+		return nil, err
+	}
+	if existingByUID != nil {
+		return existingByUID, nil
+	}
+
+	if email != "" {
+		existingByEmail, err := m.getByEmail(ctx, email)
+		if err != nil {
+			return nil, err
+		}
+		if existingByEmail != nil {
+			return existingByEmail, nil
+		}
+	}
+
 	row := m.pool.QueryRow(ctx,
 		`INSERT INTO users (firebase_uid, email)
 		 VALUES ($1, $2)
@@ -48,7 +94,33 @@ func (m *UserModel) FindOrCreateByFirebaseUID(ctx context.Context, firebaseUID,
 		 RETURNING `+userColumns,
 		firebaseUID, email,
 	)
-	return scanUser(row)
+	user, err := scanUser(row)
+	if err == nil {
+		return user, nil
+	}
+
+	// If a concurrent request inserted by email or firebase_uid first,
+	// read the existing record and continue without surfacing a 500.
+	if isUniqueViolation(err) {
+		existingByUID, readErr := m.getByFirebaseUID(ctx, firebaseUID)
+		if readErr != nil {
+			return nil, readErr
+		}
+		if existingByUID != nil {
+			return existingByUID, nil
+		}
+		if email != "" {
+			existingByEmail, readErr := m.getByEmail(ctx, email)
+			if readErr != nil {
+				return nil, readErr
+			}
+			if existingByEmail != nil {
+				return existingByEmail, nil
+			}
+		}
+	}
+
+	return nil, err
 }
 
 func (m *UserModel) GetByID(ctx context.Context, id string) (*domain.User, error) {
diff --git a/frontend/src/pages/subscribe/Subscribe.jsx b/frontend/src/pages/subscribe/Subscribe.jsx
index 4a342a2..98e44e7 100644
--- a/frontend/src/pages/subscribe/Subscribe.jsx
+++ b/frontend/src/pages/subscribe/Subscribe.jsx
@@ -40,8 +40,14 @@ export default function Subscribe() {
   useEffect(() => {
     const payment = searchParams.get("payment");
     const sessionId = searchParams.get("session_id");
+    const signupLockKey = sessionId
+      ? `kp_onboard_signup_started_${sessionId}`
+      : null;
 
     if (payment === "cancelled") {
+      if (signupLockKey) sessionStorage.removeItem(signupLockKey);
+      sessionStorage.removeItem("kp_onboard_email");
+      sessionStorage.removeItem("kp_onboard_pw");
       setSearchParams({}, { replace: true });
       setStep(2);
       setError("Payment was cancelled. Please try again.");
@@ -53,15 +59,27 @@ export default function Subscribe() {
     // Phase 1: no account yet — create it. Auth state change remounts the
     // component (App.jsx swaps route trees) and Phase 2 runs on the next mount.
     if (!currentUser) {
-      if (!userProps.email || !userProps.password) {
+      const savedEmail = sessionStorage.getItem("kp_onboard_email");
+      const savedPw = sessionStorage.getItem("kp_onboard_pw");
+      if (!savedEmail || !savedPw) {
+        if (signupLockKey) sessionStorage.removeItem(signupLockKey);
         setSearchParams({}, { replace: true });
         setError("Session expired. Please start the signup process again.");
         setStep(1);
         setLoading(false);
         return;
       }
+
+      // Guard against duplicate signup attempts caused by repeated effects.
+      if (signupLockKey && sessionStorage.getItem(signupLockKey) === "1") {
+        setLoading(true);
+        return;
+      }
+
       setLoading(true);
-      signup(userProps.email, userProps.password).catch((err) => {
+      if (signupLockKey) sessionStorage.setItem(signupLockKey, "1");
+      signup(savedEmail, savedPw).catch((err) => {
+        if (signupLockKey) sessionStorage.removeItem(signupLockKey);
         setSearchParams({}, { replace: true });
         setError("Account creation failed: " + err.message);
         setStep(1);