Can I force users to make unique e-mail addresses in django-registration?
django-registration has several forms included in the source – one is a RegistrationFormUniqueEmail, which might help you ...
P.S. You can adjust the form to use by changing the default backend or by implementing a custom one, where you return the appropriate form class, see: http://bitbucket.org/ubernostrum/django-registration/src/073835a4269f/registration/backends/default/init.py#cl-118