Each name has to be unique within a group, so when renaming an idproperty, one
has to make sure that the parent group does not contain duplicates afterwards.
This patch raises a `NameError` when setting the name to one that exists
already. Alternatively, one could delete the already-existing property, but that
seems unexpected and the user should rather do that explicitly.
This also adds a new unit test for this case.
Pull Request: https://projects.blender.org/blender/blender/pulls/146892
4d91f5c8ef