I just got off the phone with iTunes support and the option of "Display Duplicates" has been removed from iTunes 11.0. He also informed me that there is no supported way via them to revert to a previous iTunes version (i.e. 10.0+) but there may soon be someone on-line with an idea to do such or possibly there is one already from a previous change to iTunes. The disclaimer said only do I agree to terms, not what we are going to remove add in the new version. Don't ever upgrade?
There is also 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.