The UI already indicates with the exclamation point whether a drop is something that you don't currently own, but it would also be useful to know how many you have if it's something that you already own, since at some point there's no real reason to own more -- e.g. even if you run a party of three dwarf warriors, a tenth copy of a weapon or a fourth copy of a dwarf skill isn't very useful.
mouseover the item and you see the count in the top right corner of the tooltip, note that it does not count the item in the chest itself