Q: whats the fastest way to remove duplicates from my iTunes library not one by one

Posts by turingtest2 about different types of duplicates and techniques - https://discussions.apple.com/thread/3555601 and https://discussions.apple.com/message/16042406#16042406 (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/message/25867873#25867873

Show exact duplicates (Mac and Windows) - https://discussions.apple.com/message/16951281#16951281

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.