django queryset get only one result for one column pair

I have a field 'submission' which has a 'user' and a 'problem'.
How can i get a django queryset search result which will give a list
of only one result per user-problem pair.

--
You received this message because you are subscribed to the Google Groups "Django users" group.
To post to this group, send email to django-users@googlegroups.com.
To unsubscribe from this group, send email to django-users+unsubscribe@googlegroups.com.
For more options, visit this group at http://groups.google.com/group/django-users?hl=en.