title: ProgrammingError: column specified more than once
body: <p>I'm attempting to run 'syncdb -all' in my django project, having just added this model.</p>
  
  <p>However I'm getting this error message:</p>
  
  <pre><code>django.db.utils.ProgrammingError: column "target_content_type_id" specified more than once
  </code></pre>
  
  <p>Why is this happening when 'target_content_type_id' is not repeated and not in any other models?</p>
  
  <pre><code>class Action(models.Model):
      actor = models.ForeignKey(User)
      verb = models.CharField(max_length=500)
      action_object_content_type = models.ForeignKey(
          ContentType, related_name='action_object', blank=True, null=True
      )
      action_object_object_id = models.CharField(
          max_length=500, blank=True, null=True
      )
      action_object = generic.GenericForeignKey(
          'action_object_content_type', 'action_object_object_id'
      )
      target_content_type = models.ForeignKey(
          ContentType, related_name='target', blank=True, null=True
      )
      target_content_type_id = models.CharField(
          max_length=500, blank=True, null=True
      )
      target = generic.GenericForeignKey(
          'target_content_type', 'target_content_type_id'
      )
  
      public = models.BooleanField(default=True)
      timestamp = models.DateTimeField(auto_now_add=True)
  </code></pre>