Any way to display the duplicated songs in iTunes 11?
Apple have put the "Show duplicates" function back to iTunes 11. Go to View menu in iTunes and select "Show duplicate items"

A word of caution though, iTunes shows many false duplicates for me, anything with the same track name appears to be shown as duplicate. I much prefer the way Tune Sweeper deals with duplicates, there's no false duplicates and you can choose to keep the highest quality track.