Duplexing is just one of those things that are neither fish nor foul.
Interesting how duplexing became twisted totally backwards from how it began.
In the early days of smokeless, the primers that worked so well with black(they were much weaker than primers we know today) would not give a consistent ignition with smokeless, so what is a shooter with a thousand berdan primers to do? He dumps 2-3 gr. of black in the case and then finishes charging with the new fangled white powder. And hallaluah, he's back in the shooting business with his trusted old rifle and the new stuff some of the cool kids on the block are using.
