(no-ticket) Use distinct in visible_contests

We noticed using debug toolbar, that the results from the query
come from the database duplicated.

I did not investigate exactly, where it came from, but this
should not hurt.

Change-Id: I21282dd491c124162e03bc36df871f171043ae6d

oioioi/contests/utils.py

@@ -238,7 +238,7 @@ def visible_contests(request):
         controller = controller_class(None)
         visible_query |= (Q(controller_name=controller_name) & controller
                           .registration_controller().visible_contests_query(request))
-    return set(Contest.objects.filter(visible_query))
+    return set(Contest.objects.filter(visible_query).distinct())
 
 
 @request_cached