the default registration form doesn't check if the email was used in the past. So registration more than one account with the same email address is possible.
That's not good, because you can reset you password only with the email address.
My work-a-round is to add form_class with registration.forms.RegistrationFormUniqueEmail in urls. See: https://github.com/jedie/djangobb_project/commit/80e1d69749238aba66889e267ac8319ba06df689
IMHO this form should be the default form for user registrations.