When you use the spy rat mechanic and you have a singular item in hand, you are able to dupe that item in a stack.

For example, I have a netherite block in the rat's inventory, I just simply shift-rightclick and it dupes into a stack.

I posted video in #rattiest-gang on your discord, I would advise to look at that for further showcasing. I hope this gets patched soon, thank you so much !