If I place a block, then exit, kill the game, then go back in, I will have the placed block, and another one in my inventory. This happens with any blocks. Most people use this to get unlimited diamonds of something.

What I thought would happen, is the placed block would be there, but not another one in my inventory.

What really happened was I saw the placed block, but another one was in my inventory.

1. Place a block in a game
2.Close the game, then kill Minecraft
3.Go back to the game, and the placed block will be there, but the same block in your inventory.