I tend to have multiple songs located across different playlists and more especially when someone else uses the PC, they tend to modify their playlists by moving songs from other playlists etc.

• Would be nice to go through a collection of songs and see whether they are already in existing playlists
• Would be nice to go through say the collections database and find out whether a song is in a playlist or not
• Can help with managing duplicates
• Can help in managing very large playlists or a lot of very small playlists
• Can help in sorting and managing an existing playlist
• Shouldn't be too difficult to implement as the information resides within the database
• Feature exists in iTunes
• Helps simplify importing tracks from other playlists or other applications
• Helps in managing playlists for singles that aren't part of albums.  For example, 12" vinyl only tracks or remixes

Lastly, just wanted to say amaroK is an awesome player!  Well done the developers!