It sure would be a hell of a job doing, but you could write something that compares every post to "todays active topics," or at least a list of posts made today,
Man, I sure wouldn't want to be the guy that had to write it. And it sure would slow down posting, having to compare every new post against every post made today. Every post would be slower than the last.
I suppose you could to it only on new topics but then wouldn't you also want to check someone spamming replies?