Dev is aware, don't worry.
He said he didn't want people to be able to sell items, because that could possibly screw up the economy.
I have over 10k in my pocket, so selling 20 hats for 200 or so is not gonna boost me very much.
But, equally, the low financial gain is irrelevant, so may as well just delete them, anyway.
I think it's more for people starting out/early game. If you can farm clothes to sell, then maybe some of the purchases for clothes with Gracie, or wine with Isabel, become too easy. IDK.
Either way, he's aware of the issue around dropping duplicates, so hopefully this will be fixed with next update.