This is pretty standard for web logins. You first check to see if it exists and offer to email the "forgotten" password if found. If they are just "guessing" and it's not their account, they won't get the password. So the "check" is really a FIND before ever creating a new account. You may consider other means to assure the proper credentials are used for a login (txt the person's phone with the validation code, offer questions and answers, etc.)
Because account names must be unique in FM, running a script to create an account when that account name already exits will not create a duplicate account, but will generate an error.
Beverly's suggestion about testing with a Find and allowing an email to the valid email account holder sound like pretty standard components of such systems where new users can register themselves.
In fact, sending credentials to the entered email is better than just creating it and letting the user right in. They might be entering a dummy email that doesn't exist, so you still want them to have valid access to the email they entered or not let them in.