The show duplicates feature is now under the View menu. Use Shift > View > Show Exact Duplicate Items as this is normally a more useful selection. You need to manually select all but one of each group to remove. Or use my DeDuper script if you don't want to do it by hand. Please take note of the warning to backup your library before deduping. See this thread for background.
(If you don't see the menu bar press ALT to show it temporarily or CTRL+B to keep it displayed)