Help removing duplicates in iTunes playlist

I have a BIG (80GB) playlist in iTunes containing all my favourite bands which I update periodically when new albums are released etc. To save time (so I don't have to go through each band's folder in Finder and drag in just the albums that I've downloaded since I last updated the playlist) I just multiple-select all the bands in my Albums folder that I want to appear in the playlist and then drag them all in.

Unfortunately, doing this has added not only the albums new to the playlist but also all the albums that were already in the playlist again, thus giving me duplicates of the majority.

I've tried organising the list by date added and removing all the originals, but it doesn't work. I've tried clicking 'show exact duplicates' but all this does is show me the duplicates AND the originals, so I'd still have to command-click through every other file to get rid of one of them.

Is there a quicker way to go through this entire list selecting just ONE of these duplicates for deletion??