I don't see a problem with duplicate titles, for a lot of reasons... but if you do not want them --
You can check for duplicate titles the way Alex suggested, but you might want a more permanent solution.
What about making the Title field "Unique"
That would prevent people from entering duplicate titles, but you'd need to make sure you did the right error checking to tell them that they need to pick a different title.
So many sites have the same titles, though, this might create more problems than it solves.
The URL should be unique, but that prevents duplicate listings (That's ok, since you can put a link into alternate categories).
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