Scenario
User encounters UNIQUE_CUST_ID_REQD error code with the following details "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" when adding Customer record via SuiteTalk with Auto-Generated Numbering enabled. However, upon checking in the NetSuite UI, there is no customer record with the same customer ID, first name and last name.
Solution
The error encountered above is caused by two SuiteTalk SOAP request that is adding Customer records have the exact same timestamp. NetSuite is trying to assign the same Auto-Generated number to both records as its customer ID which causes one of requests to fail with the error. To avoid this from happening, make sure that there are no SOAP requests that are sent with the exact same timestamp specially when adding new records.