Whoops - my newbie status confirmed - I now see that this is already incorporated not by a check mark but by dimming. That works for me! Please delete this suggestion.
[I'm a new user - and loving the program but also noting a few enhancements I'd love to see including: When adding to an existing Playlist, songs already included should have the check mark so I can instantly see what songs are already included. I've gotten used to this assist in my prior music reader - and feel it would be a wonderful enhancement to Stage Traxx 3. This is how the initial Playlist is created (songs are checked when added) - why not show the existing selected songs when adding to a Playlist.]