Using the Skillet in Farmer's Delight with Deployers in order to expediate the campfire cooking of Slimy Fern Blend into Gunpowder causes item duplication. The process I went through is as follows:
Attach encased fan over magma block to power a Deployer above a FD Skillet, with a hopper going into the Deployer, place several stacks of Slimy Fern Blend into the hopper. The Deployer will place a stack onto the Skillet which will begin outputting gunpowder to the right of the Skillet which is then collected by another Hopper, but each time the Deployer touches the Skillet, there will be a full stack in the Skillet again, but the Deployer won't actually use any items up in the process, causing infinite gunpowder to be generated since the Slimy Fern Blend never runs out.
Setup is visible here:

Note: This can be fixed by updating Farmer's Delight to version 0.5.4 in the next version of the modpack