ValueError: Cannot insert a node which has already been saved.

During shallow copying, sometimes we seem to attempt to insert a node that has already been saved. Possible that this is a result of repeated application of a copy operation?

Sentry Issue: STUDIO-B88

ValueError: Cannot insert a node which has already been saved.
(4 additional frame(s) were not displayed)
...
  File "contentcuration/db/models/manager.py", line 351, in copy_node
    batch_size,
  File "contentcuration/db/models/manager.py", line 385, in _copy
    can_edit_source_channel,
  File "contentcuration/db/models/manager.py", line 531, in _shallow_copy
    self.insert_node(node_copy, target, position=position, save=False)
  File "mptt/managers.py", line 72, in wrapped
    return method(self, *args, **kwargs)
  File "mptt/managers.py", line 522, in insert_node
    raise ValueError(_('Cannot insert a node which has already been saved.'))

Task duplicate_nodes_task[ce3d48ec86a447acb58b4210f0b641c6] raised unexpected: ValueError('Cannot insert a node which has already been saved.',)

Start with the Sentry trace and contentcuration/db/models/manager.py at copy_node, _copy, and _shallow_copy, then inspect mptt/managers.py at insert_node. Reproduce or trace duplicate_nodes_task and determine why a saved node reaches insertion; done means the shallow-copy path no longer raises this ValueError.