You can choose to hide your collection, tradelist or wantlist, but I don't think you can selectively choose to hide what's listed.
I don't actually think there is a reason to either to be honest. If you have them, you have them, and I don't know of anybody who ever uses that section other than to see if somebody might have one to trade (if they are looking for it) or if it appears on a wantlist (if they have one to trade).
Many people, myself included, consider their duplicates to be trading stash and nothing more, but some people want to have as many copies as possible of some cards. There's no wrong way to do it.
VERY slow trading due to health problems. Not transferrable so safe to trade with, just moving is painful and can't always access the cards.
Cardboard History My COMC
New Collection Website: Cardboard History Gallery (Still under construction)
Tips on how to make your scans look like the card does in hand (No more washed out, fuzzy scans!):