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I am so sorry that I am not posting videos, please forgive me, my IPad doesn’t allow me to do so.
1. Turn off wifi of turn on airplane mode
2. Go on Minecraft PE
3. (Optional) Turn on peaceful (It’ll be easier)
4. Select the block you want to duplicate and place it down quickly
5. Go to home screen
6. Double tap home button and delete Minecraft pe from the box below
7. Go on Minecraft PE
8. Go to your world and you (might) see your inventory still has the block and the block is still where you have placed it.
Go to youtube for much more clearer reference.