Eli is spot on... I would only add that duplicate Contact or Leads in SFDC can cause havoc in eloqua. For duplicate Leads or Contacts (2 of the same Lead or Contact), I'm not 100% on this but I think that the record that wins the update going back in via autosynch, has something to do with which has the greater character count. Basically don't have duplicate Contacts or Leads in SFDC or life is not quite as awesome as it could be.
Now if you have both a lead and the contact and the same corresponding fields are getting updated then the data priority order comes into play, as well as your filters for those autosynchs and the timing.