@gvso, That condition is triggering the second message for me. as attached in screenshot.
As we are checking this condition if (!$this->isRegistrationDisabled()) { Which means, If registration is enabled and we are not able to login new user we display error message.
Which is right !! I could see why we have this error messsage as you raised the question. I missed that while creating a patch. But, then how do we avoid two error messages?
I think we Should we add a check here - if (!$this->isRegistrationDisabled() && $drupal_user) { Because, $drupal_user can be NULL and It could be null only if we have an error on - \Drupal\social_auth\User\UserManager::createUser Which means we already have an error message why $drupal_user does not hold an Object.