title: Remove duplicates from django list_filter
body: <p>I'm using a list filter in django admin on a field in a related object. </p>
  
  <pre><code>class A(models.Model):
     #..
     pass
  class B(models.Model):
     fk = models.ForeignKey(A)
     val models.CharField(max_length=1)
  </code></pre>
  
  <p>In the admin for <code>A</code>, I'm trying to <code>list_filter</code> on <code>B__val</code>, but the result is duplicate <code>A</code>'s listed for each <code>B</code> satisfying the filter value.</p>
  
  <p>Is there an easy way to intercept the query result to remove the duplicates?</p>