user_external_login_register performs not (always) valid registration

We are developing an authentication module that uses the user_external_login_register function for loading the account or register the new one if it's necessary.

One of our user has reported (and he's absolutely right) that it's possible to register a new user with an already used e-mail address witch case an unwanted situation in the user database. For example these user accounts cannot be changed later until the e-mail address isn't changed to a unique one. The situation is the same with the user's name what should be unique too.

Further details can be found at the issue #399968: shib_auth can create accounts with identical emails.

The problem can also be reproduced with our module or with any other that uses this function.