Answer
This is a confirmed current limitation in Aurena bookmarks — a bookmark can only belong to one group at a time, and adding it to a different group moves it rather than duplicating it. If this is something you need, raising it as an idea on IFS Ideation (and getting other users to upvote it) is the way to push for it being added in a future release, since it isn't planned as of the versions this was checked against.