First, Marketo should not create a duplicate lead when a form is submitted with an email address of an existing lead, it should update the existing lead. Are you using embedded Marketo forms on your website, non-Marketo form using server-side form post or a non-Marketo form using the SOAP / ReST API? When the duplicate lead is created on the second form submission, is that second form submission at the same time or within a minute or two of the first form submission?
To send an alert to sales whenever a form is submitted create a smart campaign with a Fills Out Form trigger and a 'Not Lead was Created in past 1 minute' filter. The flow would use the Send Alert flow action to send to the lead owner.
There is already a Duplicate Leads smart list in Marketo where you can see duplicates. When you say you want to set up a smart campaign to look for duplicate form submissions, what action do you want that smart campaign to take - send an alert email about a duplicate record being created?