FirebaseAuthUserCollisionException?
<p>I am using the firebase documentation to use Facebook login my android application. I am successfully able to login users using Google and Twitter. But when i click on Login with Facebook, the button is changing to Logout button. Actually it should redirect to Login success activity, because i configured <code>FirebaseAuth.AuthStateListener</code>.</p>

<p>I am posting the <code>Activity</code> here, Please let me know if i am doing anything wrong.</p>

<pre><code>public class MainActivity extends AppCompatActivity implements GoogleApiClient.OnConnectionFailedListener {

    private static final String TAG = "MainActivity";
    private static final int GOOGLE_RC_SIGN_IN = 9001;
    private static final int TWITTER_RC_SIGN_IN = 140;

    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener mAuthStateListener;

    private GoogleApiClient mGoogleApiClient;
    CallbackManager callbackManager;
    @BindView(R.id.google_signin_button) SignInButton mGoogleSigninButton;
    @BindView(R.id.facebook_login_button) LoginButton mFacebookLoginButton;
    @BindView(R.id.twitter_login_button) TwitterLoginButton mTwitterLoginButton;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        TwitterAuthConfig authConfig = new TwitterAuthConfig(getString(R.string.twitter_consumer_key), getString(R.string.twitter_consumer_secret));
        Fabric.with(this, new Twitter(authConfig));
        FacebookSdk.sdkInitialize(this);
        setContentView(R.layout.activity_main);

        ButterKnife.bind(MainActivity.this);

        GoogleSignInOptions gso = new GoogleSignInOptions.Builder(GoogleSignInOptions.DEFAULT_SIGN_IN)
                .requestIdToken(getString(R.string.default_web_client_id))
                .requestEmail()
                .build();
        mGoogleApiClient = new GoogleApiClient.Builder(this)
                .enableAutoManage(this,this)
                .addApi(Auth.GOOGLE_SIGN_IN_API,gso)
                .build();

        mAuth = FirebaseAuth.getInstance();
        mAuthStateListener = new FirebaseAuth.AuthStateListener(){
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();
                if (user != null) {
                    //User is signed in
                    Log.v(TAG, "Yo Baby");
                    Log.d(TAG,"name"+user.getDisplayName());
                    goToHome();
                } else {
                    //Do some logic
                }
            }
        };

        mGoogleSigninButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent signinIntent = Auth.GoogleSignInApi.getSignInIntent(mGoogleApiClient);
                startActivityForResult(signinIntent, GOOGLE_RC_SIGN_IN);
            }
        });

        callbackManager = CallbackManager.Factory.create();
        mFacebookLoginButton.setReadPermissions("email", "public_profile");
        mFacebookLoginButton.registerCallback(callbackManager, new FacebookCallback&lt;LoginResult&gt;() {
            @Override
            public void onSuccess(LoginResult loginResult) {
                handleFacebookAuthentication(loginResult.getAccessToken());
            }

            @Override
            public void onCancel() {

            }

            @Override
            public void onError(FacebookException error) {

            }
        });

        mTwitterLoginButton.setCallback(new Callback&lt;TwitterSession&gt;() {
            @Override
            public void success(Result&lt;TwitterSession&gt; result) {
                handleTwitterAuthentication(result.data);
            }

            @Override
            public void failure(TwitterException exception) {

            }
        });

    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);

        if (requestCode == GOOGLE_RC_SIGN_IN){
            GoogleSignInResult result = Auth.GoogleSignInApi.getSignInResultFromIntent(data);
            if (result.isSuccess()) {
                GoogleSignInAccount account = result.getSignInAccount();
                handleGoogleAuthentication(account);
            } else {
                //Google Login Failed
            }
        } else if (requestCode == TWITTER_RC_SIGN_IN) {
            mTwitterLoginButton.onActivityResult(requestCode,resultCode,data);
        } else {
            callbackManager.onActivityResult(requestCode,resultCode,data);
        }
    }

    private void handleGoogleAuthentication(GoogleSignInAccount account) {
        AuthCredential credential = GoogleAuthProvider.getCredential(account.getIdToken(), null);
        mAuth.signInWithCredential(credential).addOnCompleteListener(this, new OnCompleteListener&lt;AuthResult&gt;() {
            @Override
            public void onComplete(@NonNull Task&lt;AuthResult&gt; task) {
                if (task.isSuccessful()) {
                    //Save Credentials in Google Smart Lock
                } else {
                    //
                }
            }
        });
    }

    private void handleFacebookAuthentication(AccessToken accessToken) {
        AuthCredential credential = FacebookAuthProvider.getCredential(accessToken.getToken());
        mAuth.signInWithCredential(credential).addOnCompleteListener(this, new OnCompleteListener&lt;AuthResult&gt;() {
            @Override
            public void onComplete(@NonNull Task&lt;AuthResult&gt; task) {
                if (task.isSuccessful()) {
                    //Save Credentials in Google Smart Lock
                } else {

                }
            }
        });
    }

    private void handleTwitterAuthentication(TwitterSession session) {
        AuthCredential credential = TwitterAuthProvider.getCredential(session.getAuthToken().token, session.getAuthToken().secret);
        mAuth.signInWithCredential(credential).addOnCompleteListener(this, new OnCompleteListener&lt;AuthResult&gt;() {
            @Override
            public void onComplete(@NonNull Task&lt;AuthResult&gt; task) {
                if (task.isSuccessful()) {
                    //Save credentials in Google Smart lock
                } else {

                }
            }
        });
    }
    private void goToHome() {
        startActivity(new Intent(MainActivity.this, HomeActivity.class));
        finish();
        return;
    }

    @Override
    protected void onStart() {
        super.onStart();
        mAuth.addAuthStateListener(mAuthStateListener);
    }

    @Override
    protected void onStop() {
        super.onStop();
        if (mAuthStateListener != null) {
            mAuth.removeAuthStateListener(mAuthStateListener);
        }
    }

    @Override
    public void onConnectionFailed(@NonNull ConnectionResult connectionResult) {

    }
}
</code></pre>

<p><strong>Update1</strong></p>

<p>Finally I understood the reason why Facebook login is not working. This is because i already logged into the app using Google Signin. Since my email for Google and Facebook are same it is giving <code>FirebaseAuthUserCollisionException</code>. I should have Debugged the application before i post the question in Stackoverflow.  </p>

<p><strong>Update2</strong></p>

<p>I am not closing this question as it may help someone who faced the situation like me. And I will also update the solution for <code>FirebaseAuthUserCollisionException</code>, as i dig more into it.</p>