Remove all duplicates on iTunes library
Solution 1:
You can use the default iTunes engine to find duplicates :
File > Display Duplicates
Or, use a third-party software like TuneUp or TidySongs.
Solution 2:
"Show duplicates" been removed from iTunes 11. As we are left with having to use 3rd party apps to remove duplicates now, Tune Sweeper is the one I use.