Overview:
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).
Root Cause:
The Parent Account on the Child Account object should be read-only. Reparenting of Child Accounts is not supported.
Solution:
Set the Parent_Account_vod to Read Only on the page layout of the Child_Account_vod object.
Note: If the Parent_Account_vod field is made a Master-Detail field on the Child_Account_vod object, it cannot be made Read Only at either the Object or Page Layout level. The solution in this scenario to prevent this error occurring is to ensure that Users have access to both the Child Account records and the Parent Accounts for any Child Accounts.
Related Documentation:
CRM Help Documentation: