Please Disable picking up an item you already have

Me as well as many others enjoy setting up a little "shop" in some secluded area of the map where we farm the goblins for items, and stockpile them on the ground. I enjoy making my shop area a "violence-free zone" where nobody kills the other team, and i trade items with ANYONE on either team. they give 1 item in exchange for another. there is always some moron who doesnt get it and runs in, taking EVERY SINGLE ITEM I had on the ground. You should not be able to pick up items that you already have in your inventory as this is also a problem where someone with full health and a health pot in their inventory will steal a potion that low-health teammate was going for, deleting the item forever. there is no good reason that this change should not be implemented.