Support for Duplicate emails in Submit form API

Hi Everybody,

We are migrating to the new  Submit form API method from the old Post method for our Marketo forms hosted on non-Marketo Landing pages. https://developers.marketo.com/rest-api/lead-database/leads/#submit_form 

This API has a seemingly unsolvable problem it does not work with duplicate emails. Because an email needs to be passed in, and it fails if there is more than one lead with that email. These emails are legitimate duplicates in our system, Is there is a way to update a duplicate with this API?