IMO, a good way to make this less frequent would be to add a 'suggest' feature for similar URLs: When someone submits a URL (or title?) what is very close, but

IMO, a good way to make this less frequent would be to add a 'suggest' feature for similar URLs: When someone submits a URL (or title?) what is very close, but not equal to another that has been submitted recently, warn the user. Allow him to continue anyway with an extra click when none matches.