Discovered this by complete accident while doing something else.

Process

Building a machine like so (2 encased fans, 3 chutes), while being powered (96 RPM is optimal), can produce up to 2 stacks a second of items. The output is the very top chute with the encased fan next to it.

It takes 2-3 stacks of items to start duping I believe. The input is the bottom chute. You drop items in there to kick start the process.

Theory

I have a theory on what's going on, although this may well be wrong, and I can't be bothered to go through the code to find out.

I believe that, when the fan is pushing an item out of a chute, it physically creates that item in the world. It then proceeds to fall back down into the chute. In the middle of this, we have a fan that's blowing the created item away from falling back into the chute. This somehow duplicates the item, making it so that the chute system is still full but we are also taking items out of it.

Maybe the chute creates the item in the world but doesn't remove the item from it's internal inventory? I have no idea.

Other Comments

On world reload, the machine can break and the chutes will need to be replaced.

Minecraft Information

Minecraft: 1.16.5
Forge: 36.0.14

Mods I have installed:
Aquaculture 2: 2.1.12
Create: 0.3e
Citadel: 1.5.3
Rats: 7.1.0
Just Enough Items: 7.6.1.71
Optifine: 1.16.5 HD U G6

If more information about either my computer or anything else is needed, I will be happy to provide it.

Duplicate of #939