What you will need:
-A USB storage device or cloud storage
-A world with a chest
-A character
-An item to duplicate


I will be referring to your original world as "world 1" and your duplicated world as "world 2"


Start by having a properly formatted USB drive or cloud storage available for use, but without any Terraria data on it. Keep everything on the hard drive for now.

-Open your world (world 1), and store any item(s) you would like duplicated in a chest. You can fill as many chests as you like, but items on your person will not be duplicated.

-Once the chest(s) are filled, save your game, exit, and go to the dashboard.

-At the dashboard, scroll to the far right tab, open settings > memory > HDD > Terraria.

-COPY your WORLD data to your second storage device (usb or cloud storage)
MOVE your CHARACTER data to the same storage device.
-Load Terraria again, this time select the storage device you just moved everything to. Using the same character you moved over, open the world you had put your items in (world 2).

-Take all the items you wanted to duplicate out of the chests, and move them into your inventory.

-Save the game, exit, then dashboard.

-Go back to the memory settings on the far right, then MOVE your CHARACTER data back to the HDD with the original world (world 1). You can delete the copied one (world 2) on the USB or cloud if you'd like.

-Load up Terraria again. Open the original world (world 1), and all the items you wanted duplicated should be in the chests, AND in your inventory!