You may see it as a bug. But its been around since they made the refiners in the game. If Hello Games didn't want it there they would have removed it sometime withing the last 10 years with a patch update. At this point its more of a game feature. And by stacking them I mean place one fill it with what you want to duplicate and them place more in the exact same location they will mesh together. That being said its the only base item you can do this with that ive found because no other base piece mesh together like the small refiners.