[MC-116196] Block duplication with slime blocks, observers, and pistons

For some reason, I got pistons and slime blocks to interact in a way that it duplicated blocks.
http://imgur.com/a/NyBM5

Related issues

pokechu22 2017-04-16T19:19:58Z

Do the duplicated blocks disappear when you right-click them? Then this is MC-54026.

pokechu22 2017-04-16T20:06:46Z

Do they drop when you break them? Or just disappear?

migrated 2017-04-16T20:09:16Z

They disappear when you leave the world and open it again, correct?

migrated 2017-04-16T20:16:21Z

Actually, they do disappear when you right click them, my bad.