It can be frustrating if you want to add a track your playlist but you are unsure if you have already added it to one or more of your playlists.
A solution would be to provide visible feedback, if the track is already in any other playlist, whenever one is moving a adding a track to a playlist. For example, while dragging a track to a playlist, all playlists that already include the track could light up in green. This would make it easy to see if you have already added the track to one of your playlists. Another benefit would be that you can avoid the “already added to this playlist” warning because you can see beforehand if a track is included in the playlist. Avoiding this pop-up would improve the overall efficiency of adding tracks to a playlist as these additional clicks would not occur anymore. Improving the visibility of the system status as well as increasing efficiency of use were shown to benefit the overall UX, which is why this change should be implemented.