Oh, give me a break. Yea, sure, there's definitely a lot that's hard to tell from a first date but to think that someone is inherently incapable of understanding enough about someone to know that they will not be marrying them after spending a few hours with them is asinine. I'll agree that in a situation where you like some things about them but the date just didn't go well, you should go out again, but to say you always have to go out twice is ridiculous.
You mention 2 cases, one where you "know" that you won't be marrying someone after a few hours, and where you like some things but it "didn't go well".
What percentage of ppl who want to reject is the 1st and what is the 2nd?
I disagree that at 20-something with little relationship experience, you can "know" with absolute certainty that someone isn't for you after a few (usually awkward) hours.
If that's your attitude, by all meant don't go out again, as you'll just hurt her by going on a "pity" date!
BUT, If you are willing to properly examine your thoughts with an open mind, and not take this "knowing" as absolute, understanding that that things you "know" can and will change as you get to know someone better, than you owe it to yourself and to her to go out again.
It's not second-guessing yourself, it's an understanding that even when you FEEL that your mind is made up, that's not always the case. (This is also an essential skill in marriage IMO).
I don't know how many times you've has bad 1st dates, but the above is written from experience.
If the problem is that you have conflicting values by all means don't go again, and try to get better research next time to minimize those encounters.