2 of 2 people found this helpful
Yes, Marketo will create new records (leads) for every new lead or contact you upload and create in SFDC, even if that same contact already exist in SFDC, but also if this recors exists as a lead in Marketo only.
The fact that you create them as lead in SFDC first will drive the exact same result. What you need to do is to avoid to create duplicates in the first place.
It is better to upload, if you can, new leads in Marketo and push them to SFDC. On list import, unlike SFDC Sync, Marketo will always dedupe the import against existing entries in Marketo.