Duplicates Elimination
How do I eliminate all my duplicates at once, rather than one at a time?
MacBook Air, iOS 8.1.2
How to find and remove duplicate items in your iTunes library - http://support.apple.com/kb/HT2905
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).
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
http://dougscripts.com/itunes/itinfo/dupin.php (commercial)
iTunes Duplicate Song Manager - http://sourceforge.net/projects/itunesdsm/
http://www.hardcoded.net/dupeguru_me/
http://www.wideanglesoftware.com/tunesweeper/index.php
http://www.araxis.com/find-duplicate-files (commercial, free trial) - finds duplicate files on computer, not specifically iTunes
I would do it manually. As you can see from turingtest2's post, some duplicates are not really duplicates.
I would also review how you add media to iTunes. It should not be producing all that many duplicates.