I'm about to use one of those multi-provider authentication frameworks Opauth or Hybridauth (don't know which is the best but I had a little preference for the second one) so the consequence is that I have to manage multi account.
In fact I already have my own account management (login+password+email) and I had last year facebook users. Now I will have to deal with Twitter account, linkedin account, etc... So the question is easy : how to manage the username ? For instance, John create a account on my site. So "John" login/username is taken. But if another John from Facebook then another one from Twitter arrive, what i'm supposed to do ? In Stackoverflow (or elsewhere), what is the practice ?
I don't want a John write on the behalf of another John. Today with my own system, I verify that the login (username) is unique. If not I reject the user. But now ?
I thought that I can check the email but... I learnt that email address can't be the key because of Twitter that doesn't give it to you.
Hope I'm clear enough :)
regards