This is one of the many duplication glitches on mcpe that I have seen.

Materials:
Chest (at least 1open space near top)
Item to duplicate (I will use a diamond)

Time:~15sec

Step 1: Chest

Open the chest.

Place the items in the chest 1 at a time.

Close the chest.

Reopen the chest.

Take the items out 1 at a time.

Step 2: Wait

Wait 12 seconds without moving!!!!

Step 3: Restart Game

Completely shut off the game (not just the home button) and turn it back on.

Step 4: Check

Turn the game back on and go to the world you should have the items in your hand (if you don't you counted too fast) and the chest (if you don't you counted too slow)