Rekordcloud can perform health checks on your library. With these you can easily fix things and create order and structure in your library.
Tracks without playlists
This will scan your library for any tracks aren't found in any playlist. This is ideal for DJs that have all their tracks organized in playlists. Any tracks found here can be exported into a new playlist.
Duplicate tracks in playlists
This will scan your all your playlists for any duplicate tracks. You can then see which tracks are found multiple times in which playlist and choose to remove them if you want.
Note that duplicates are based on their reference to a track. This means that two tracks in two different locations with the same artist and title are still considered different tracks.