Is the issue that you do want duplicate leads, or is that you do not want to change owner.
If you just use Sync Lead to SFDC: Auto Assignment Rules, then Marketo will NOT Change Owner on an existing SFDC Lead.
It sounds like you had it move to a Queue directly, which is what will happen. Instead, you can try
- only have SFDC use Auto Assignment, do not have Marketo move someone. Let SFDC do it.
- check for Queue ownership before attempting to sync.
- send an alert to Lead Owner when the person is already owned, instead of moving the lead.