In API, when creating a user with a same email but different userID, API returns 404 with "wrong user id supplied" error which is fine. (though the error message is not right). But we can override this by creating a new user with a new email and new userID and updating email to someone else's email. then we can have two users with two userIDs but same email.
You can apply this for name either.
This is not happening from this web portal as a email uniqueness validation stops it.