We have Demand Tools for SFDC, which has routines running, but there are a few gaps, as identified by the Marketo Possible Duplicate list.
I like the idea on the alert, however the way our business operates, sales isn't creating leads, rather Marketing is.
Also, we recently implemented a method in SFDC that will alert the Sales team, in Salesforce, that a record with that email is already in existence. Not sure on speicifcs as that was our CRM team who implemented it.