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