If a lead and a contact exist in SFDC with the same email address and a list is uploaded into Marketo with that email address. Which record in SFDC will the Marketo record sync to?
The SFDC Type is not important. Nor is being in SFDC at all.
The list upload will merge into the most recently updated Marketo person.
In case you are dealing with duplicate records often, you may find this blog post on the community that discusses the implications and the behavior of the Marketo system with regards to the duplicate records helpful. 🙂