Item Duplication Glitch

I was messing around in creative world on my redstone world, And I decided to take a trapped chest and an item hopper and connected the item hopper to the chest and threw in a piece of TNT(At the time I was making a TNT cannon). I looked in the chest and I saw 2 pieces of TNT. I then tried this on my survival world with some stone and it duplicated the stone as well.