[mukyu]

There is nothing specific about how Diablo 2 did items that enabled duping. Nearly every MMORPG has had bugs that allow duping. You are working with distributed systems with tens if not hundreds of ways of interacting with your inventory and have to make sure they all work properly even when an attacker can crash your servers at any time during the processing. This all has to be done while at the same time allowing massive amounts of transactions to occur simulatenously. It is not an easy thing to deal with.