As a partial solution, how about adding a polite note on the "submit" page, asking people nicely to check the past few days' worth of front pages before submitting, to see whether there has already been a discussion on the same subject.

I've just been to the "new" page, where I flagged a few versions of the "Internet Explorer Users Are Stupid" story from yesterday.

And it wasn't a very good story the first time (though it's exactly the sort of thing which tends to get a lot of upvotes, for entirely the wrong reasons).