Django 1.5 - admin update user, got message "User with this Username already exists" (Django 1.4 excepted update of the user)

Django 1.5 - admin update user, got message "User with this Username already exists" (Django 1.4 excepted update of the user)

Created a simple polls app, and using the django.contrib.auth in INSTALLED_APPS

When an attempt is made to update a user (in the admin frontend) this message is produced: 'User with this Username already exists.'

The same example is working fine (update the user without warnings) in Django 1.4

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