Just an idea, if this was added...
It might be useful if the message indicated whether the duplicate track being added has already been played, or is queued to be played later.
I'm thinking of scenarios where people use MusicBee to play music at a party say, where adding a track that has been played already might be a deliberate choice, as opposed to inadvertently adding a song that is going to be played eventually anyway and then hearing it twice.
This would be easy to check for normal playback, but might be too complicated in shuffled order.
I don't really know how MusicBee keeps track of what has already been played from the Playing Tracks list to guess.
Bee excellent to each other...