When I made a elevator using double piston extenders, and I activate it to push slime blocks up, the slime blocks and some sticky pistons got duplicated and it happens randomly every time.

At first, there were 2 slime blocks but after they got pushed up, another one appeared. I went saved and quit about 5 times for the glitch to stop. Sometimes it even generate invisible blocks and as I ride the elevator when it goes up about 20 blocks, you fall out of the elevator because blocks just get directly duplicated ontop of you.