AD provider + DNN provider + Email as username = Duplicate emails

All my sites have "use email as username" option enabled. This requires emails to be unique, but AD authentication provider still allow to create user with email that already used by another user.

Generally this is not a problem, but it could lead to some unexpected behavior - e.g. after that I could not set default authentication provider via site settings - then I try to do so, it shows error message about duplicate emails and just does nothing.

I think it would be better if AD provider would check for duplicate email for newly created users if DNN provider and "use email as username" both enabled. In case duplicate found, email field could be set to blank and error message displayed/logged.