Player 1 fills in a recipe in the crafting table and it shows a final outcome, at this point Player 2 opens the crafting table and takes the final outcome. For both players, the materials are consumed and both players are able to grab the final outcome.

Confirm it works, I'm playing with him and we were crafting machines and found out the dupe bug when I intercepted his item while it was being crafted.

Can confirm, tested multiple times, some blocks and items from ae2 such as interfaces, patterns, crafting storages, storage discs , GT wires are all examples of infinitely dupable items.
Other items such as machines are dupable once so 1 = 2

What mod is causing this behavior?

@Damien-Reave I think RealBench is causing this problem, it allows other players to see your crafting process. You might want to contact the developer of this mod, and in the meantime remove the mod from the modpack til it is fixed

Removed RealBench for 1.0.5 but I can't test if that changes anything in my single player testbed world.

I'll leave the report open so people can test on servers if removal of RealBench fixed things.

In 1.0.5 it's even easier to duplicate items:

I launched a server with RealBench only, and have Minecraft only run RealBench aswell. The 1.0.4 exploit is caused by RealBench, I do have no clue what happened in 1.0.5 that made duplication even easier, but removing RealBench will solve the duplication exploit.

Actually... in 1.0.5 you can duplicate as many items as you want.

I notice that RealBench-1.12.2-1.3.3 wasn't actually removed in the 1.0.5 update, which is probably why the glitch still persists.

Yea same with issue #429