In my solutions i almost always use email as user identifier which means there can only be one user associated with an email address. This would mean if someone used someone elses email it would indeed result in 'this email is already in use'.
However, the real owner of the adress could easily get a password reminder to his email.
Solution to faking email addresses; Send out an account activation email to the email specified to be able to finalize the registration process.
In what context does your problem / question arise?