Maybe it’s very simple, but I didn’t figure this out: How is it possible to create a new tag with the same name as an existing group?
You could exclude the existing group from tagging (e.g. via the Info panel or the contextual/action menu) and add a new group/tag to the “Tags” group.
That works, thanks.
I found another way of doing it: Create a new tag and rename it. It can then have the name of an existing group.