There is no way to distinguish the user name from the email if I can enter a user name identical to the email.
Then there are sites that forbid me to enter my email as user name
forbidding characters, for example, such as "@".
These sites are diabolical because they force me to invent a username that is not taken and that I can remember as if the password was not enough. At least I can take a nice password without fear. Then, if I forget my username, I have to recover it also as a password?
The least evil sites at least allow me to enter my email address with the name of the user at least that is the one I remember.
In my opinion, the user names must die!
The best thing for your users to do is to let them sign with an e-mail only. Simple and clean.
There is too much waste of Usernames without any clear advantage over Email – here is my more detailed answer on the subject