Fixed #15322 -- Removed a redundant check in admin logins. Thanks to melinath for the report.

        if username and password:

            self.user_cache = authenticate(username=username, password=password)

            if self.user_cache is None:

                if username is not None and u'@' in username:

                if u'@' in username:

                    # Mistakenly entered e-mail address instead of username? Look it up.

                    try:

                        user = User.objects.get(email=username)