Place the item or coins to be duplicated in a chest in any world (it will not affect the items in it afterwards). Run the "TerrariaServer.exe" file in "C:\Program Files\Steam\steamapps\common\terraria". Select the world you put the item in on TerrariaServer.exe. Join a multiplayer game, and type "localhost" in the IP page. Take all the items you want to duplicate using your character. Then, type "exit-nosave" at the console on TerrariaServer.exe. It will disconnect your character, but it will not save the world. Thus, your items will still be as they were before you went in. Go to the desired world, and unload all your duplicated items. Repeat this as many times as desired. Note: This only applies to worlds you can make by yourself with your own TerrariaServer.exe, and will not work on anyone else's world. This glitch was tested on version 1.1.1.
Note: This glitch only works in Single Player mode in versions 1.1 or earlier; it was patched in version 1.1.1. You need a music box and two wooden platforms. Place the two platforms two squares from the roof of your house. Then, place the music box on the platform, and the music box will explode into hundreds of boxes. Simply sell the extra boxes for easy money.