Salesforce is already doing a fine job stopping duplicated leads from being created through web-to-lead forms.
The only problem is that the only notification is a "Salesforce Could Not Create This Lead" email to the administrator.
I would like to notify the owner of the lead/contact (or even a static email if that's easier!) when this happens. Ideally, I would like to use a custom email template because the current notification is not easy to digest.