I have try real duplex loads in a modern 12 ga once, iirc, 10 grains unique( what I had then) under 70 grain of cheap blackpowder.
" cheap blackpowder" means one of the brands your barrel looks like a hundred yours old stovepipe after the first shot. I learned from the dublex experience that the bottom powder in a shell will care for the dirt the upper powder is making.
think I load here 15 grains Swiss 2 FFg and top it with 65 grains czech Fg powder. barrel looks surprisingly clean with this, not like nitro but for blackpowder excellent.

btw, next year not with the Collath but the mother of all shotguns if everything will be the ok then