Without you being a little more verbose in your requirements it's hard to say this is exactly what you're looking for, but have you taken a look at the Duplicate Email Checker?
BTW, all I did to find that was click on "Addons" at the top of the page, specify Vanilla1 and plugins and bung "email" in the search box...
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BTW, all I did to find that was click on "Addons" at the top of the page, specify Vanilla1 and plugins and bung "email" in the search box...