Three chances to hit an enemy per shot, pre burst-fire clockwork on automatic rifles.
We had one; the 7.62 NATO Duplex Ball. Just two bullets in a .308 case, but the same idea.
Mike is correct. It's a concept that comes up periodically, and some folks are still playing with it. This outfit is still playing with the concept. And, Corbin makes swage dies for a stacking bullet. From what I've seen over the years, the results have never been truly satisfactory.