I did have a search for a topic which may shed some light on this but couldn't find anything. If there is one I do apologise.
I very recently updated to iTunes 11 (no idea how long ago it came out but I generally take a while updating iTunes) apart from the fact I hate the new design, and search, I'm having a bit of trouble with the display duplicates function. Basically I can't find it and it looks like it's been removed.
I did a bit of searching through help etc and found the following link but I swear that option doesn't display in my File drop down menu.
Is it just me? I found this function really handy for helping organise my files and it seems bizarre to have removed this option.
MOD EDIT: It' has been restored in 11.0.1 under the View menu.