Why "await Firebase.initializeApp()" called twice?

[rikai-trongpq]

Following your example, https://github.com/FirebaseExtended/flutterfire/blob/master/packages/firebase_messaging/firebase_messaging/example/lib/main.dart

Future _firebaseMessagingBackgroundHandler(RemoteMessage message) async {
// If you're going to use other Firebase services in the background, such as Firestore,
// make sure you call initializeApp before using other Firebase services.
await Firebase.initializeApp();
print('Handling a background message ${message.messageId}');
}

/// Create a [AndroidNotificationChannel] for heads up notifications
AndroidNotificationChannel channel;

/// Initialize the [FlutterLocalNotificationsPlugin] package.
FlutterLocalNotificationsPlugin flutterLocalNotificationsPlugin;

Future main() async {
WidgetsFlutterBinding.ensureInitialized();
await Firebase.initializeApp();

// Set the background messaging handler early on, as a named top-level function
FirebaseMessaging.onBackgroundMessage(_firebaseMessagingBackgroundHandler);

if (!kIsWeb) {
channel = const AndroidNotificationChannel(
'high_importance_channel', // id
'High Importance Notifications', // title
'This channel is used for important notifications.', // description
importance: Importance.high,
);