What about login issues?

Question

Login can only be applied when there is only one piece of data in the data, multiple data will cause errors

Answer 1

There is no error in matching username and password with only one piece of data, unless your usernames are the same. If they have the same username, it is theoretically unscientific. Check whether the usernames are the same when registering. If they are the same, it is Registration is not allowed