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 where Referer validation depends on header being present
This lab's email change functionality is vulnerable to CSRF. It attempts to block cross domain requests but has an insecure fallback.
To solve the lab, use your exploit server to host an HTML page that uses a CSRF attack to change the viewer's email address.
You can log in to your own account using the following credentials: wiener:peter