Apple has released an update to iTunes 11 that fixes several issues with the new software and brings back detection of duplicate library items.
This update to the new iTunes addresses an issue where new purchases in iCloud may not appear in your library if iTunes Match is turned on, makes iTunes more responsive when searching a large library, fixes a problem where the AirPlay button may not appear as expected, and adds the ability to display duplicate items within your library.
You can get the update by selecting Software Update from the Apple menu at the top left of your screen.
This update to the new iTunes addresses an issue where new purchases in iCloud may not appear in your library if iTunes Match is turned on, makes iTunes more responsive when searching a large library, fixes a problem where the AirPlay button may not appear as expected, and adds the ability to display duplicate items within your library.
You can get the update by selecting Software Update from the Apple menu at the top left of your screen.