This glitch allows you to duplicate a Use Item as many times as you want, until you are out of room for any more items.

#1. Select a party member that has their Boost Gauge filled.
#2. Open the item menu and select the item you wish to duplicate.
#3. At the second party member, hit B to return command back to the previous party member.
#4. Select Boost from the first party member (the one that used an item).
#5. Press B to toggle Boost off.
#6. Repeat Steps 4 and 5 until you receive the desired amount of items.
#7. Finish the battle as normal. Credit goes to Kai Powell.