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If the lead in SFDC had already synced down to Marketo, the system will update the existing record (which could mean overwriting data in fields if they aren't blocked from updates). If the lead in SFDC hasn't synced into Marketo, then a new record will be created in Marketo (or an existing, unsynced lead is updated) and, after hitting a Sync Lead to SFDC flow step, will create a brand new Lead record in Salesforce containing the data stored in Marketo.
Adding to Steven -
If the lead hasn't synced back to Marketo - the list import would create a new record in Marketo and sync to SFDC will create a new record in SFDC - potentially creating a duplicate. This would happen only if the lead was created in SFDC just after the latest sync and hasn't synced yet as the next sync would be due in few minutes.
If you have a lead in SFDC which has synced to Marketo, your import will update the data. If your have a lead in SFDC but hasn't synced yet, it'll create a duplicate. Hope this helps!