only add item to list once

oml/user/models.py

@@ -254,7 +254,7 @@ class List(db.Model):
         from item.models import Item
         for item_id in items:
             i = Item.get(item_id)
-            if i:
+            if i and i not in self.items:
                 self.items.append(i)
                 if self.user_id == settings.USER_ID:
                     i.queue_download()