I think it is better to add +1 to the last id for a new id.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@19364 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/Core/Folder.py

@@ -193,7 +193,7 @@ class FolderMixIn(ExtensionClass.Base):
       Id exist, then save the Id.
     """
     try:
-      my_id = int(self.getLastId())
+      my_id = int(self.getLastId()) + 1
     except TypeError:
       my_id = 1
     while self.hasContent(str(my_id)):