UNIQUE_CUST_ID_REQD error on upsertList

The upsertList method updates or creates new records depending if the external ID specified in the web service request is already used.

Take note that all other field values, other than the external ID, that are part of the request, are implied to set or update the value of their corresponding fields.

One of the fields that could be set on the customer record is the entity ID (Customer ID). When setting the said field make sure that the value being set to it is not yet used on any other record.

Setting entity IDs that are already used would result to a UNIQUE_CUST_ID_REQD error.

In the user interface, upon editing a customer record, entering an already used enity ID will result to an error :

"A customer record with this ID already exists. You must enter a unique customer ID for each record you create. To correct this record, click back and enter a new customer ID in the Customer field. Then, click Submit."

which is the same error that will be received in SOAP request.

Remember that the behavior of Web Services is patterned to that of the user interface.