Does iTunes 11 need extra services or tools to manage duplicates in the library?
I found 160 duplicates with dupeGuru.

But, none of them can be found in iTunes, even if they are in the hard drive.

Look: I have two identical copies of the same song on my hdd (same artist, same bitrate, same duration, different album), but when I search it in iTunes I can't find it.



It looks like iTunes found duplicated songs and removed them from the library (without removing them from the hard drive, though)

Maybe that's why tools that scan the iTunes library file won't find duplicates, while tools that ignore the library file and scan the folder will find a lot of them.

I arrived to the conclusion that iTunes will "hide" songs if it thinks that it's an exact duplicate.

More proof. I got two identical songs from iTunes store in 2010 (The same song in two different albums)
One of them is on the HDD but can't be found in iTunes



Or, again:



But, when there are too many differences, like 1 second in duration, or an "à" that become an "a", the entry is kept:



That's why Apple "removed" the "find duplicates" menu: it's done silently and automatically - no need to search for them. (Though, the file gets "lost" in the HDD, wasting space...)