The contents of chests can be duplicated, as long as they only contain two textboxes. Usually, textboxes have a 10 frame buffer after them to prevent opening the save and quit menu. By mashing through the text quickly enough, this buffer does not exist. If you pause on the first frame after the textbox disappears and save and quit, the game will register you as having obtained the contents of the chest but not mark the chest as having been opened. This allows you to open the chest again, obtaining a second copy of the item. This can be done multiple times in a row.
Progressive items that are duped will get their next phase. For example, duping the bow will give you the next quiver upgrade.
This is notably used to dupe small keys in Tower of Spirits 4. It is also used in 100% to dupe the Regal Ring and easily obtain rupees. However, the latter does not work on NA, due to the Regal Ring having three textboxes in that version, preventing you from saving and quitting soon enough.
Regal Ring Duplication
The Regal Ring that you obtain at the Trading Post has another duplication method. Get the Regal Ring, save and quit, and then navigate to the DS treasure trading menu and place it in one of the boxes. Go and talk to Linebeck and complete the cutscene where the Bridge Worker "takes" the Regal Ring as payment. You can then return to the trading menu and take the Regal Ring out, allowing you to keep it.
This can only be done once, unlike the above method, but it is incredibly consistent. It can be done in combination with the above method to obtain multiple Regal Rings in 100%.
