You will need 2 things. 2 accounts, and a hacking client. Login to your client with both accounts. Go to servers, pick USMIDWEST or any other populated server, have some valuable items (Cbow, Dbow, Acropolis armor, Dblade, A.S.S, Cdirk ect) and whichever account has the items you want to dupe Go to options, and change your Interact/buy key to H, stands on the guild hall. THE SECOND ACCOUNT TRADES THE ONE THAT IS ON THE GUILD HALL. Click the item you wish to dupe, go onto the other client, and accept from the account with no items. Once you accept on that account, go to the account with the items, and right when you hit accept on it, spam the H key. You should Disconnect and get sent to the main screen, thus dupes the items.

Proof it works: