Minecraft Version: 1.12.2
Forge Version: 14.23.5.2847
MagneticCraft Version: Magneticraft_1.12-2.8.3-dev

Steps to reproduce:

1. Get a fabricator.
2. Place it down.
3. Place the block you want to duplicate in any number of slots on the left side.
4. Place the block on the right hand side also
5. Double Click on the item in the right hand side.

Proof (Video/Gif):
https://i.imgur.com/DYE0ZUN.gif

Yeah can confirm it's an issue that i am having the same issue.