Let's suppose the following: a user created a new profile with email/password authentication, then they filled in their profile information to be saved their profile, and then the operation was performed. But what will happen once they log out and want to log back in again, and find that OAuth button on the login screen? The normal behavior is that they will simply want to use the different OAuth-based authentication methods instead of adding their email/password again.
Choosing one of the many present OAuth authentication options will result in a problem. A new account will be automatically created, and that means more work for the user. From here, we come to the idea of linking multiple accounts into one.
This means that a given user will link all their OAuth authentication profiles into one. This would imply...