I've been shooting a Shiloh Sharps in 45-90 for 35 years and never had to duplex a powder charge. My load is 80 grs., drop tubed and a bullet with plenty of grease capacity. The Lyman 457125 is a great example. Basically most Lyman bullets are as is the RCBS 500 BPCR, (I think that's right). In my experience, once a charge goes over 80 grains fouling control becomes more of an issue and I use a grease cookie. I also use one if I can't find a bullet that carries enough lube, usually it's smaller caliber cartridges. If shooting matches a blow tube works wonders.

I do duplex the loads for my 11.15 X 60R double rifle. It's BP and the only way I could achieve enough velocity for it to regulate was to start with 5 grs. of SR-4759 under 68 grs. of Ffg. It shoots extremely clean.

For shotgun shells I do as 3DogMike does, saturate the fiber wad. I've shot sporting clays rounds of 25 birds and never wiped the bore.