It's already pretty common to require clicking a verification link in an email to complete registration. If you're using someone else's email on the registration page, you're already doing something fishy, and this prevents you from discovering that the email is already in-use. To a legit user, everything looks normal (and to a legit user who forgot they had an account, they'll get a simple reminder in their inbox when they look for their validation link, possibly with forgot password instructions included).

Of course, this all goes out the window if you're not using emails as usernames.