Allow multiple accounts for same email address

Just use a different email address that goes to the same place. Plus addressing should work.

Both of these:
fake+user@example.com
fake+user2@example.com
Should go to fake@example.com.

Plus addressing works on most email systems I’ve used. But test it yourself first.

But, if you really really must disable this safety check, edit wp-includes/registration.php and look for function email_exists( $email ). Change it to always return false and you’re good.