Follow the following steps to duplicate any item (or coins) (Tested on Ver 1.1.1)
1.Place the item you want to dupe in a chest in a world(can be any world, it wont affect the items in it after)
2.Run the TerrariaServer.exe( can be found in "C:\Program Files\Steam\steamapps\common\terraria")
3.Select the world you kept the item on on the TerrariaServer.exe)
4. Join multiplayer game and type "localhost" in the ip page.
5.Take all the items you want to duplicate using your character.
6.Next, on the console for TerrariaServer.exe, type exit-nosave (it disconnects your character but doesnt save the world, so your items will still be as it is before you go in.)
7.Go to desired world and unload all your duplicated items.
8.Repeat steps to do it over and over again.
This only applies to worlds you can make by yourself with your own TerrariaServer.exe, won't work on anyone elses world.