Posts by turingtest2 about different types of duplicates and techniques - https://discussions.apple.com/thread/3555601 and https://discussions.apple.com/thread/3293239?answerId=16042406022#16042406022 (Note: The DeDuper script is for Windows).
http://dougscripts.com/itunes/scripts/ss.php?sp=scanfordoubleentries
http://dougscripts.com/itunes/itinfo/dupin.php (commercial) There are other similar tools, particularly a much earlier version of this package called "Corral iTunes Duplicates", which I am sure a general web search would reveal.
E.g., Corral iTunes Duplicates mod. Eric Pugh - http://opensourceconnections.com/blog/2006/11/11/better-itunes-song-deduping/
May 2014 post on iCloud duplicates - https://discussions.apple.com/thread/6253706?answerId=25867873022#25867873022
Show exact duplicates (Mac and Windows) - https://discussions.apple.com/thread/3555601?answerId=16951281022#16951281022
I use 2 procedures:
1) I only generate about one duplicate every 2 years. Change how you use iTunes to stop making duplicates.
2) I always deal with duplicate manually, regardless of how many. Sure there are automated ways, if you trust them.