Not to be arrogant, but I don't think I've ever been significantly "humbled" ... I didn't get into the Ivy-league colleges I applied for, I guess, but that wasn't really humbling. This is tough, and the response can't overlap too much with the "most difficult obstacle" prompt. I don't think I'll have a super fast turnaround for Duke's secondary.
I actually had a similar first impression of this question. But a humbling experience is definitely not the same as an experience of failure, or dealing with obstacles, etc. Humbling implies an experience that knocked you down a peg. That doesn't even mean that you failed, it just means that the experience rendered you more modest/less egocentric. Service is an excellent in-road to answering this question. E.g. (and not my topic) by serving the less fortunate, your interactions teach you humility and empathy; you recognize that, despite the apparent external differences, plenty of what transpired in their lives that lead them to this point is bad lack. You realize your own fortune, and in doing so, you become less aloof, less dignified, and
more human. And so on. (this is obviously not well-written, I'm just trying to create an example of a humbling experience). Everybody applying to medical school should be able to knock themselves down a peg or two and be able to display modesty and humility. This is essentially a chance to do that.
This essay took me a very long time, as did the all of them, to be honest. Good luck!
EDIT: I'm just another pre-med kid applying this cycle, so my advice should be taken with a (large) grain of salt. If you have a different way of answering this essay, that comes more naturally to you, then you should pursue that angle instead. The only requirement for these essays (beyond answering the prompt, obviously) is to show your character.