Ensures that missing UPNs are not marked as duplicates.
The second commit made an exception for when the childIDs of the duplicate UPNs are the same. However, this too should not be allowed as each child should appear only once in the childIdentifiers table.
The second commit made an exception for when the childIDs of the duplicate UPNs are the same. However, this too should not be allowed as each child should appear only once in the childIdentifiers table.