Django 1.3 authentication
I will try to answer why this was happening for us, i worked on it long time ago so will try to recollect as much as i can. 
We were allowing admins to modify the login id of the user, This would go and change the email id int he partial digest table. A lot of times they would use this to disable an account by changing the login id of that user. Now what would happen is this user who's not able to login as his id is changed did a trial registration with us using the same email id/password as before and hence now the partial digest table will have two entries.
We have digest authentication in our application. For some reason we are seeing for a few users having different id, username as in "auth_user" table but for some reason in the django_digest_partialdigest the user_id is different but the "login" column has the same username. 
I Am not able find out what scenario would lead to this kind of entry in the db.
we allow signup/activation of account/resetting password.