Currently Being ModeratedDec 11, 2012 6:24 PM (in response to Tnbirdman)
The show duplicates/show exact duplicates features have been left out of iTunes 11. Rumor suggests they will be restored in the next build. In the meantime I have written two Windows scripts to make playlists of Duplicates and Exact Duplicates, either from a selection of tracks or the entire library. Note that, as with the iTunes feature, this list makes no distinction between "originals" and "dupes", you have to decide which is which.
There is also my DeDuper script for automatically removing duplicate copies but keeping one remaining copy of each set. This can preserve ratings, play counts, playlist membership, etc. which are lost in a manual clean up. Please take note of the warning to backup your library before deduping. See this thread for background on deduping and the script.
If you want to manually remove duplicate tracks use shift-delete to remove selected tracks from the library as well as the playlist. Keep one of each repeated group of files and don't send the others to the recycle bin unless you are sure that there are multiple files on the disc as opposed to multiple entries to the same file. Same advice to backup applies.