问Distinct()方法不起作用吗？

EN

 public ObservableCollection<string> CollectTopicsFromXml()
    {
         ObservableCollection<string> oc = new ObservableCollection<string>();
         XDocument xDoc = XDocument.Load(path);
         var topicColl = xDoc.Descendants("topic").Distinct();


         foreach (var topic in topicColl)
         {
             oc.Add(topic.Value);
         }

        return oc;
    }