DOC: add explanation

src/caoscrawler/sync_node.py

@@ -87,6 +87,9 @@ class SyncNode:
 
         parents are added if they are not yet in the list
         properties are added in any case. This may lead to duplication of properties.
+        We allow this duplication here and remove it when we create a db.Entity (export_entity
+        function) because if property values are SyncNode objects, they might not be comparable (no
+        ID, no identifiable) yet.
         """
 
         if other.identifiable is not None and self.identifiable is not None: