Problem: If you have multiple customer records in Shopify (different customer IDs) which share the same company name and email, NetSuite will not allow records to have the same customer ID.
Solution: Enable auto-generated numbers on your customer records. (NetSuite customer ID would just be the company name if you don't have the auto-generated number enabled). By using auto-generated numbers, NetSuite will put in a unique customer ID for every customer record created, instead of using just the Name (see example below):
Note: You may also check NetSuite documentation regarding auto-generated numbers for further information.