During the 1950s, the US Army ran a series of programs trying to find a better solution for infantry rifles than firing single semiauto bullets. These projects (including SALVO, SALVO II, and SPIW) would include experiments with multiple barreled rifles, burst firing rifles, flechette firing weapons, and multiplex cartridges. That is, single cartridge cases with two or three bullets stacked inside. The idea was that a soldier would have a single sight picture, trigger press, and recoil impulse – but would have multiple chances to hit the target.

https://www.forgottenweapons.com/duplex-22-06-m1-garand-at-ria/