Social auth displays two error messages

In a decoupled application, when we rely on third party api to create user.

The create user flow has a minor change: Using social auth we first create user on third party tool and on success of that api call, we want to create user on Drupal and associate a customer id from that third party api to drupal user.

We have implemented this by subscribing to event SocialAuthEvents::USER_FIELDS. So, once we have the user fields from social auth provider, we make an api requests to third party api, But if third party api fails we throw an error in event subscriber. And when we throw an error it displays two error messages.

Here is what happens,

// \Drupal\social_auth\User\UserAuthenticator::authenticateUser
public function authenticateUser($name, $email, $provider_user_id, $token, $picture_url = FALSE, $data = '') {
     ......... 
     ........
    // At this point, create a new user.
    $drupal_user = $this->userManager->createNewUser($name, $email, $provider_user_id, $token, $picture_url, $data);
    $this->authenticateNewUser($drupal_user);
    return $this->response;
 }

while performing createNewUser, It calls - \Drupal\social_auth\User\UserManager::createUser and inside that method, There's a try..catch

try {
      // Initializes the user fields.
      $fields = $this->getUserFields($name, $email, $langcode);
      ........ 
      ........
     return $new_user;
} 
catch (\Exception $ex) {
      $this->loggerFactory
        ->get($this->getPluginId())
        ->error('Could not create new user. Exception: @message', ['@message' => $ex->getMessage()]);
}

$this->messenger->addError($this->t('You could not be authenticated, please contact the administrator.'));
return FALSE;

that's why, when we throw error from getUserFields it will catch the error without creating user and shows first error message You could not be authenticated, please contact the administrator..

Then, as shown in the first code snippets it will call authenticateNewUser with $drupal_user Null
And as the $drupal_user arg is null, it will add another error message to display error message. You could not be authenticated. Contact site administrator.

  public function authenticateNewUser(UserInterface $drupal_user = NULL) {
    // If it's a valid Drupal user.
    if ($drupal_user) {
       // process based on drupal user object.
    }

    if (!$this->isRegistrationDisabled()) {
      $this->messenger->addError($this->t('You could not be authenticated. Contact site administrator.'));
    }

    $this->nullifySessionKeys();

    $this->response = $this->getLoginFormRedirection();
  }

Hence we have two error messages here.