Flutter 如果其他Provider 的帐户已存在，如何阻止用户登录

流程步骤:

1. 登录Twitter mail@gmail.com(成功)
2. 登录Google mail@gmail.com(成功重写Google的提供程序)
3. 再次登录Twitter mail@gmail.com(失败，帐户已存在于不同的Provider )

我对第二步感到很困惑..因为，Firebase不会阻止登录到GoogleProvider 并重写Provider

Future<Either<Failure, Unit>> signInWithTwitter() async {
    try {
      if (kIsWeb) {
        await _firebaseAuth.signInWithPopup(_twitterAuthProvider);
      } else {
        await _firebaseAuth.signInWithProvider(_twitterAuthProvider);
      }

      return right(unit);
    } on FirebaseAuthException catch (e) {
      return left(
        Failure.authFailure(
          message: e.toString(),
        ),
      );
    }
  }

Future<Either<Failure, Unit>> signInWithGoogle() async {
    try {
      final googleUser = await _googleSignIn.signIn();
      if (googleUser == null) {
        return left(const Failure.authFailure());
      }

      final googleAuthentication = await googleUser.authentication;

      final authCredential = GoogleAuthProvider.credential(
        idToken: googleAuthentication.idToken,
        accessToken: googleAuthentication.accessToken,
      );

      await _firebaseAuth.signInWithCredential(authCredential);

      return right(unit);
    } on FirebaseAuthException catch (e) {
      return left(
        Failure.authFailure(
          message: e.toString(),
        ),
      );
    }
  }

那么，如果已经存在其他Provider (如Twitter、GitHub、Facebook等)的帐户，如何阻止用户登录？