Importing and Serial Number in source file indicates already in use
1) Check IML to make sure they are on supported version.
2) Have them navigate to the IM404_TransactionTierDist file and note the Transaction Entry number.
3) Next check the IM_TransactionHeader and IM_TransactionDetail to see if the transaction number exists there.
4) If it does, then chances are there's a valid entry in Transaction Entry for that serial number
5) If not, have him delete the record from IM404_TransactionTierDist, then Re-initialize the IM404_TransactionTierDist file only and try the import again.
It could have been that the VI job created an orphaned record in that file.