This is a bug with gmail: User can use one google email to register multiple accounts. Details: my email is email@example.com. I can use my email to register new account with this tricks: insert dots between the name of the mail. For example, firstname.lastname@example.org, or email@example.com. Xenforo understands they are separate emails, but actually any mails to firstname.lastname@example.org come to email@example.com.