title: Duplicate key issue when loading back.json file PostgreSQL
body: <p>I have a PostgreSQL database that where I performed python manage.py dumpdata to backup the data into a json file. I created a new PostgreSQL database, performed a migrate, and everything worked like clockwork. When I tried to load the backup.json file with python manage.py loaddata backup.json it, gives me this error.</p>
  
  <pre><code>Could not load contenttypes.ContentType(pk=15): duplicate key value violates unique constraint "django_content_type_app_label_76bd3d3b_uniq"
  DETAIL:  Key (app_label, model)=(navigation, navigation) already exists.
  </code></pre>
  
  <p>I checked phpPgAdmin, and there is a row for News. Is there a way to load the backup json file without including the content types, or better yet dump everything except for content types data ?</p>