https://www.quora.com/What-is-a-duplex-shell-or-duplex-round-Are-there-any-benefits-to-using-them
“A duplex cartridge is one where there are two bullets. It was made under the theory that the two rounds will have paths that slightly diverge as the fly and increase the chances that one will hit the target. And if they both hit, they create two wounds instead of one.”
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One brand that makes such is Double Tap. Test have shown that at anything more than close range the two parts diverge more than is desirable.
So, with a binary trigger and duplex ammo you get 4 slugs down range with one trigger pull?
Take my money!