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