This is one of the many duplication glitches on mcpe that I have seen.
Chest (at least 1open space near top)
Item to duplicate (I will use a diamond)
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)