The show duplicates feature in iTunes 11 is now under the View menu. Select the Songs view (or one of the List views) and use Shift > View > Show Exact Duplicates as this is normally a more useful selection. You need to manually select all but one of each duplicate group to remove. You also need to consider whether you have duplicate files or just duplicate entries in the database. In the first case you want to trash the files for the tracks you're deleting, but not in the second.
(If you don't see the menu bar press ALT to show it temporarily or CTRL+B to keep it displayed)