How could it be checked if somebody has more than one account? You even don't have to confirm your email address to signup. And even if you did, there are tons of disposable email services (more so, opening an email account on, for example, gmail is also not a trouble - you can easily have tens of emails). Blocking by IP address also can't prevent people with multiple accounts if they're mining with rented rigs... The system is easily abusable. Somebody correct me if I'm wrong...