Earlier today, I posted a question to stackoverflow. Later, I went to post another question. When I clicked "Ask Question", instead of giving me a blank question page, it gave me a page that was pre-filled with the subject line, body, and tags from my previous question. If I post, will it overwrite my last question?
It will not overwrite your previous question. Most likely what happens here is that you had your previous question open in two windows (started it, opened a new question tab that was then prefilled), submitted one and closed the other. Your question then is still cached and will appear the next time you open a new question form.
I have this happen with a mild amount of regularity when I have multiple tabs open.