title: Does db.ListProperty act like set() in avoiding duplicate keys?
body: <p>I have a model with a property called tags of type db.ListProperty(db.Key). The app will be adding new tags to the entities and it is possible that added tags already exist as part of the list.</p>
  
  <pre><code>class Entity(db.Model):
    tags = db.ListProperty(db.Key)
  </code></pre>
  
  <p>How does app engine handle those? Are duplicates avoided? Or do we have to do something like:</p>
  
  <p><code>entity.tags = list(set(entity.tags.extend(new_tags_list)))</code></p>