You cannot register an email address that is already taken by another user. If you change your own email address while testing your exploit, make sure you use a different email address for the final exploit you deliver to the victim.
Lab: CSRF vulnerability with no defenses
This lab's email change functionality is vulnerable to CSRF.
To solve the lab, craft some HTML that uses a CSRF attack to change the viewer's email address and upload it to your exploit server.
You can log in to your own account using the following credentials: wiener:peter