Definitely a useful option. I've added the following to our issue tracker:
Consider adding two duplicate detection options:
1) A way to detect duplicates within the current playlist in the playlist editor. So for example, if the same track is added twice, both instances might be highlighted in red.
2) A way to detect duplicates between multiple playlists. This would likely need to be proactively initiated by the user somehow (i.e., a "Check for dupes against other playlists" button) unlike #1 above which could unobtrusively be set to run automatically. The UI could be similar though -- tracks in the current playlist that appear elsewhere could be highlighted (perhaps in a different shade of red) and either a new column could be added, or a tooltip could be used, to indicate which other playlist(s) the track appears in.