Block Duplication with Sticky Pistons

When a sticky piston is on a clock, the block it's pushing may duplicate (may be a phantom block, but I'm not sure), and the blocks made will push other ones. When used with sticky pistons and slime blocks, blocks touching the slime block will get duplicated too.

When I observed it, I was in the nether (above the roof of the Nether) at around Y=217.