"result objects do not have unique primary keys" error when fetching randomly-sorted queryset

[tuatara]

I've had an intermittent 500 error, with this report:

AssertionError: django_polymorphic: result objects do not have unique primary keys - model <class 'mash.articles.models.Story'>

It's always been on that model, which led me to a query in a template tag that returns a random story:

stories = Story.objects.filter(suppress=False, publish_date__isnull=False, publish_date__gt=datetime.now(), byline__isnull=False).order_by('?')[:5]

If I execute that query many times in the Django shell, I eventually get that error. If I run it with the order_by clause removed, or changed to order on a field, it works reliably. It also worked reliably with normal model inheritance. So I'm thinking there's something about the way django-polymorphic treats random ordering, but a quick reading of the code doesn't flag anything for me.

This is with Postgres 8.4. Happy to share models if it would help.

Thanks Matt

[variable]

I too have this problem, to reproduce it by running this query, Article.objects.filter(tags__slug__in=['tag1', 'tag2']) if an article has been tagged by 'tag1' and 'tag2' will result above error

I guess it's the way that query has been customized to work with polymorphic, which results multiple same instance in a result set.