This KBA focuses only on Legacy picklists. For MDF picklists see KBA 2328179 - Legacy to MDF Picklist Migrations | General Information
While using the Instance Synchronization Tool and syncing picklists between instances, you may see the "External Code not Unique" error.
From the 1505 release onwards, syncing picklists requires that a unique External Code is assigned for each existing picklist option, or it cannot be successfully synched.
The external code field on the picklist file should be populated on both source and target, and it should be identical before we do the sync. The sync will consider the PicklistID + External code criteria to match the records across source and target.
External Code not unique , KBA , LOD-SF-PLT-SYN , Instance Synch , Problem