I'd really like to stress this since a few users seem confused:
It's 1 post per 24 hours.
If you posted more then once per 24 hours, you're automatically disqualified from winning and the person after you will win instead (provided they followed the rules)
It's not 1 per day, or per 12 hours.
Thanks. Does this mean that I and wwzsocki disqualified because we posted some offtopic messages yesterday?
Hope it isn't so

upd. Can somebody win more than 1 prize?
If you posted more then the rules state (1 per 24 hours) then yes.
To answer your second question, yes someone can win more then once if they're the winning post multiple times