When you are speaking of lead or steel shot duplex loads, it was just a marketing gimmick. Some considered them multi range loads but here's my take on duplex loads...
Many hunters see duplex loads as multi-range loads that are good at close range and still good at long range due to having both small and large pellets.
In reality, putting some large pellets in with some smaller ones doesn't do anything except reduce the number of each pellet size in the load. It will more than likely handle close-range shooting just fine but reducing the loads number of large pellets due to the inclusion of small pellets will not doubt limit its effectiveness as a long-range load by having too few large pellets.
My thinking is, if you are shooting birds at ranges where the smaller pellets are effective then you might as well be shooting a full load of the smaller pellets, and if you need a long-range load then reducing the number of larger pellets by taking up room with smaller pellets isn’t the way to go.
Go with the pellet size that will get you the pattern density and pellet energy you need for the bird type/size you are shooting and then choke it appropriately for the distances you shoot your birds.
Good luck!
PS: Here are some of my pattern numbers from the Remington Duplex Turkey load that showed it would be just fine out to 40 yards but, questionable beyond that. Those 49 #4 shot don't really add anything significant to the pattern.
REMINGTON SP 4X6 MULTI-RANGE DUPLEX SHOT SHELL
12 GA 3" 1 7/8 OZ #6 & #4 LEAD (#4/49, #6/336 = 385 PELLETS) 1,210 FPS
40 YARDS – REMINGTON TURKEY SUPER FULL EXTENDED CHOKE
30” PATTERN COUNT / SKULL/CV HITS
315 / 4
314 / 4
314 / 3
293 / 5
289 / 5
AVERAGE (%) / 305 (79%) / 4.2