I think you can just override the default clean_username() in RegistrationForm: https://bitbucket.org/ubernostrum/django-registration/src/27bccd108cdef30dc0a91

I think you can just override the default clean_username() in RegistrationForm: https://bitbucket.org/ubernostrum/django-registration/src/27bccd108cdef30dc0a91ed1968be17bb1e60da4/registration/forms.py?at=default#cl-45 https://bitbucket.org/ubernostrum/django-registration/src/27... Looks like that's the only place that enforces duplicate usernames.