When you have a redstone clock with a Comparator on with a lever and a redstone signal around it, it creates an extremely fast pulse. But when you take a sticky piston and put it in the ground next to a repeater, and put a slime block on top of it, and than another sticky piston/piston next to the slime block with a repeater there and make the pulsing happen it'll duplicate the pistons. The pistons don't act as a normal piston (i.e; you can't make them extend), also when in Survival Gamemode, when you break the pistons they do not appear as an entity, they just disappear as if you were in Creative Gamemode. But the strange thing is that you can stand on them as if they were a solid block, so it's not a visual error.