I find two queens in about 1 in 20 hives. Maybe a tad more.
Never really noticed it until I started using full size hives for grafting after pulling out the queen, then later grafting into the same hive.
Had me baffled the first time the bees refused to make any queens cells. I went in the hive, pulled the queen, grafted, and never even thought about two queen colonies until that first happened.
I think many beekeepers just quit after finding the first queen. And then close up the colony. Or they glance over the frame looking for a marked queen, and miss the one that is not marked.
I think it happens more than most realize.