For fast and robust handling of duplicates in any sized library you will want to check out Dupin or Dupin Lite.
You may have tracks in a Playlist (NOT the Music library playlist) that are "spawned" from the same Music library track; for example, you may have dragged or copied a track (NOT a file) two or more times to a playlist and now that track appears multiple times in the playlist. This script will remove all but one of these kind of duplicated tracks from the selected Playlist.
I get a lot of email from users who misunderstand what this script does. This script WILL NOT find and remove tracks of duplicated audio files! This script WILL NOT find and remove duplicated FILES from the Library.
Also includes French version, Purger les doublons, thanks to David Latapie.
Each of the duplicated sets of tracks in this playlist are "spawned" from the same track in the "Music" library; that is, for example, each track was dragged twice to this playlist. The script "Remove Duplicated Tracks From Playlist" will remove all but one of these tracks from this playlist. No files are deleted and no tracks are removed from iTunes.