A great feature of iOS music app is the ability to create playlists to listen depending on how you feel and what you're doing at any given time. On the other hand, sometimes you inadvertently add the same song to a playlist more than once, which means you'll have duplicate tracks on the same playlist. Because it can be annoying to hear the same song more than once, a new jailbreak tweak called Dejavu aims to solve this problem.
Dejavu prevents you add a song to the same playlist more than once in your list of "All songs" that have been previously added to the playlist.
When you try to add a song, the tweak will even show a "Song already added" indicator to let you know that the song already exists in the playlist you are trying to add it to.
Even if you are trying to do it from the playback screen while playing a song, you will get the same popup message saying "Song already added" if the track is already present in the playlist.
Dejavu adds a preferences panel in the Settings app where you can configure some options.