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Force a search for existing WorkItems before creating a new one
I've seen this working like this on Joel Spolsky discussion forums and in MS Connect.
Even though it's a bit annoying at first to be forced to search for existing items before entering a new one, this certainly prevented me on a couple occasions to enter a duplicate. In some cases, it was just a matter of voting the existing one and maybe adding a comment where I'd wish something slightly different but fundamentally the same.