This guide will tell you how to duplicate any item of your choosing in Terraria 1.4+. I recommend not doing this excessively as it can ruin the Terraria experience.
Simple Duplication Item Guide
Make sure to turn Off the AutoSave feature in settings.
- Get items you want to duplicate and leave them in your inventory.
- Save and Exit out of your world.
- Click Multiplayer.
- Click Host & Play.
- Start up a local server.
- Once in the world put the items you want duplicated in a chest and close out of the chest menu.
- Press ALT + F4.
- Load up your world in Singleplayer.
- Your items have been duplicated.
Here is a video guide on how to do this as well: