I'm not sure I understand exactly what you are trying to do but one Marketo record can ONLY be tied to one SFDC record and vice versa.
I think I understand what you are asking. We have a custom sync filter so it made things pretty complicated for us. We had multiple duplicates in SFDC, and when they were synced over in Marketo, they of course created duplicates in Marketo, and then the duplicates were deleted in SFDC, but remained in Marketo. But what I learned is that each record has it's own ID.
Marketo dedups leads based on email addresses, so usually it's a SFDC sync issue like mine listed earlier.
You can merge your records together and choose the master record (usually the most recently updated). When you merge in Marketo, they will get merged in SFDC (assuming all your leads are sycned). These are for leads that have the same email address by the way.