When updating the Parent of a Child Account in CRM offline, the user receives an error:
duplicate value found: External_ID_vod__c duplicates value on record with id: (sfdc 15-digit id).
The Parent Account on the Child Account object should be read-only. Reparenting of Child Accounts is not supported.
Set the Parent_Account_vod to Read Only on the page layout of the Child_Account_vod object.
CRM Help Documentation: