The technical term for this is superfetation. In humans, it's possible, but it's very uncommon.
Two thousand years ago, that great Greek thinker Aristotle wondered about this very topic, specifically as regards the hare.
Aristotle noticed that when a hare gave birth, quite often the offspring in a single litter would fall into two distinct classes: there would be a bunch of full-sized robust baby hares, and another bunch of scrawny frail baby hares.