How to prevent adding duplicate entries to playlists?

I run 7.9.2 with Playlist Manager v6.1. It works well. If i hear a song i want to add to a playlist, on my controller i click "+" and "Add to playlist". Now my favorite playlist is getting long and sometimes i forget if a song is already in there. If i "Add to playlist again, now there are two entries for the same song. Is there a way to prevent that? I've searched high and low, but no joy.

It would be nice if the "Add to playlist" would give an indication about that song already exists in this playlist. But i don't know if i'm asking for a new feature or overlooking something that already exists?

Anyone?