Salesforce integration - how to prevent a new record from being created

Hi, 

We’re about to integrate one of our typeforms with Salesforce. This typeform is being sent to both leads and contacts. We’re passing the email address into the typeform as a hidden field and matching the response in Salesforce via email. 

As I understand it we’re able to set up multiple mappings to both lead and contact objects, and we’re able to choose to update an existing record first, and if there’s no record to create one. 

However, my question is that because we’re gathering responses from both leads and contacts, if a response can’t be matched to a lead (but there is an existing contact) - will it create a new lead? Likewise with a contact. 

Is there a way to prevent typeform from creating a new record in Salesforce? 

I’m trying to prevent the creation of any duplicate records in our Salesforce org. 

Thanks,