I'm hoping that someone here will have a better way to do this but I did a little experiment and if no one has a better way, maybe this will work.
I operated under the assumption that you want a listing of all of your duplicate cards for a set. If you are going to try this, please read through this entire thing first because I describe a side effect near the end.
I decided to use the For Sale / Trade List as a temporary holding place just long enough to produce a list of duplicates. This doesn't work if your For Sale / Trade list already has cards from this set that you want to keep on the FS/T list. It can work if you want all of the duplicates in that list AND it can work if you don't want any duplicates from the set in that list but not if you want a mixture. In other words, if we are going to use the FS/T list as a work area for this set, we need to just use it as a work area so we can move to it and from it and not worry about keeping some things on there but not others.
I went in to the collection summary of the set. In the Chose An Action dropdown, I took Add All Duplicates to For Sale / Trade List. I refreshed so that I could see that my duplicates were separated from the rest of the cards. In my browser, the blue square with a number doesn't update automatically, not sure if that is just me, it hasn't really bothered me, just explaining why I refresh there.
The next thing that I did was on the Collection Summary for the Set, I went down to the Print Center area and I selected Print For Sale / Trade List. This gave me a report of just the duplicates.
Since I don't really want these cards on my For Sale / Trade List, I selected Move All Cards in For Sale / Trade List to move them back to the collection. If your browser acts like mine, do a refresh here to see that they are indeed back. This is the reason that you don't want to try this if you already have cards from the set in your For Sale / Trade list. This method relies on being able to move duplicates to the For Sale / Trade List, List them, and then move all of them back.
One gotcha though, I'm pretty sure the duplicates stayed on their own line, a new line in the collection after doing this. For example, I had 3 Joe Smiths, the quantity in the blue box to the left was 3. When I finished with this, there were two lines for Joe Smith, one had a 1 in the blue box and the other had a 2.
To test this, I added some basketball cards because I don't happen to collect them and I knew they would stick out to me if I failed to delete them. I also only pretended to have just a few cards total.
I personally would only do this on a set-by-set basis and I have no idea if this puts some kind of strain on the server so I would not do it frequently.
I hope this helps. Sorry for the long rambling post.
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