When browsing albums it would be so helpful if there was an indicator easy to see that shows I already added a track to one of my playlists and maybe a easy way to quickly see which playlist when i flick on the track. This would be really help to avoid adding duplicate songs. Also a notice if you do add a duplicate track saying ‘are you sure you want to add this, its already on your playlist’.
Of course you can go to a play list and search to see if you have the track on there but that takes much longer and is more cumbersome. Please consider this.