title: Can I get Django to ignore duplicate fixtures on non PK unique constraints?
body: <p>I have some base initial data that I want to include in a Django project. It fits perfectly as a fixture <em>except</em> that I might want to reject duplicates. This would mean that I could re-load the data in to an existing installation and it would only add the objects that don't already exist.</p>
  
  <p>I have used natural keys and foreign keying on the same field, so there are no questions of id clashes etc.</p>
  
  <p>Possible? I don't want to write a JSON importer when one already exists, and my data is already in a fixture.</p>