title: Using or Q() objects in limit_choices_to in wagtailadmin
body: <p>Django 1.10.5</p>
  
  <pre><code>def limit_contributor_choices():
      limit = Q(group__name="contributor") | Q(group__name="Group")
      return limit
  
  
  author = models.ForeignKey(
      settings.AUTH_USER_MODEL,
      blank=True, null=True,
      limit_choices_to=limit_contributor_choices,
      verbose_name=_('Author'),
      on_delete=models.SET_NULL,
      related_name='author_pages',
  )
  </code></pre>
  
  <p>With the following code, if a user is in more than one group, then the query returns that user multiple times. How do I get distinct values?</p>
  
  <p>I'm using this in the wagtail admin where the dropdown is generated automatically.</p>
  
  <p>Perhaps another way to look at it would be to override the queryset and add distinct()? If so, I'm not sure how to override that in wagtailadmin</p>