[Nagyman]

It's not terribly straight forward to do this, depending on which apps you may use. E.g. django-registration or even django.contrib.admin. Username is hard coded in many places. It will get there, but apps have some work to do.

[kmfrk]

I think you can just override the default clean_username() in RegistrationForm:

https://bitbucket.org/ubernostrum/django-registration/src/27...

Looks like that's the only place that enforces duplicate usernames.