To perform this glitch start with a single armor plate (not the double one).
Get the block you want to place it on and move your mouse left and right very fast, while clicking.
If done it should show it places the armor plate, but it should not appear.
Upon pressing play the armor plates will be stacked.
This is what happend to me.
(The catapult exploded and most blocks duplicated for some reason)

Ever hated that pesky spawn box that restricted the size of your beast machine?
Well no more fear!
With this you can easily build out of spawn and keep on going
Isnt it amazing?

Happy cheating, please rate this guide or share it so other people can spread the word!
EDIT:
ALL blocks duplicated upon doing this
Notice how theres 2 pilot blocks/base blocks and a duplicate of almost everything. (Yes including elastic.)