Salesforce Integration Dropping Leads

So I’ve set up the Salesforce integration and everything has been working just fine, up until now. After investigating a non-related issue I’ve found that the Salesforce integration has only been accepting leads that aren’t existing contacts in the SF database.

I’m trying to figure out if the issue is on the mautic side of the SF side. Salesforce has no issue with me creating a duplicate lead manually, however when it comes to the integration I’ve been unable to get existing customers push through.

Any help would be absolutely amazing