Use the search function there was allrdy a post on how to dupe items via TG for Elite/Unlimited/GP but afaik this is fixxed allrdy on their servers...
Try if it works on other and donīt expect that someone will give you a 100% noob friendly video tutorial
Even if its only private server, such stuff is normally not available in the public boards cause of a) ruinin the servers market to fast, b) fast and easy fixing