The google API checks on (primary) E-Mail first. So you should not have two contacts with the same E-Mail, this produces the error.
If a contact has no (primary) E-Mail adress, sync uses the other mentioned fields (Mobiule, FullName, +Company).
That's all used for the duplicate identification.
If you want to filter out / identify the contacts, how about clearing your Google contacts and...
2012-02-02 23:49:48 PST in GO Contact Sync Mod