Apple has updated iTunes to version 11.0.1 to squash a few bugs that showed up in iTunes 11. This update also brings back a feature that many users missed: the find duplicate songs function.
Courtesy of Apple
This update also takes care of the problem of the AirPlay button not appearing, and some purchases not showing up in the iTunes Library.
iTunes 11.0.1 is available via the Software Update service in the Apple menu, the Mac App Store, or directly from the Apple web site.