Usually when I've submitted a dupe story, if it already exists it just adds a vote to the pre-existing story.<p>Where the logic either breaks (or allows people to subvert it) is where different URL's can get you to the same story. Often times URL's contain some superfluous flags that don't change the content served, but just serve to log some referrer or layout type data (I'm sure everyone reading this site gets how this works). Adding, removing, or changing any of this data seems to pretty much break or confuse whatever dupe-detector logic exists.
Sounds like a nice-to-have feature. I don't think it's that bad to delete your own post if you notice the dup soon enough. No skin off your nose if the moderators kill it, either. That's happened to me.