There's a glitch to make it so that you can transfer what you're wearing on your head to multiple people. But it's compleley graphical and you don't actaully get the item. I'm not quite sure how it's done but I've seen a few screenshots.
There are no item duplicating glitches that allow the person to own and keep the duplicated item. Anybody who says there are can go find my (or my brother's) $500 challenge thread. Until then, anybody who claims that it's possible is blowing smoke.
This is an incorrect statement. I THINK the only thing that stop us from duping items is because jagex encrypted the packets that the server sends you. if you have a fast enough computer you can decrypt the packets in real time and dupe items, if you know how to do so.