I want to remove all duplicates from my library, to consider a duplicate the song must have the same encoding, bits, duration.
"Show duplicates" been removed from iTunes 11. As we are left with having to use 3rd party apps to remove duplicates now, Tune Sweeper is the one I use.
If you enable iTunes Match, duplicates will now be marked with an icon:
Note however that such files do not need to have the "same encoding" to be recognized as duplicates. Also note that (inconveniently) the file marked by Match as a duplicate (and thus "skipped") may not have the "best" encoding of the duplicated files. For example, you might see this:
In this case, in order to end up with a single song, managed by Match, with the high bit-rate song on your machine, you will need to delete both songs, leaving just the one managed by Match (the first one above)
and then download the "Purchased" song from iCloud
I haven't tried DupeGuru yet, but iDupe worked quite well for me, though I had to batch up my checkin. First I'd select iTune's buggy "display dupes" and then I'd use that selection to run iDupe on. This sped things up quite a bit. I'm interested to see whether DupeGuru improves things by checking audio finger prints.