This glitch is a better-working version of WhaleTuba's version. Please check out his profile.

NOTE: This glitch will NOT work in multiplayer

1. Put your item(s) to duplicate in a chest.

2. Take the items out and put them into your inventory, wait 1-2 seconds(this is important!!), then press Alt+f4 (Command+Q in Mac) to force quit the game.

3. When you enter back into your world, the items will be in your inventory AND in the chest!

Troubleshooting tips: -Try waiting a bit longer or shorter before force quitting the game. You want the game to not even register that the diamonds were taken out of the chest.
-If it doesn't work, try it again! Once you get the timing right, it will be very easy to do every time.

Again, check out WhaleTuba's version if you want to learn more.
I also have a tutorial on how to duplicate items in Minecraft PE. Check it out!

Hope this helped!

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