Sage have a good solution to this problem, but they have unique customer codes. By uniquely selecting a customer using the code a user can import changed customer details in a batch.  If the customer exists Sage does an update, else the customer is added using the unique, user generated 8 character customer code.  From a developers view its easy.  With Quickbooks there is the CUSTOMER field and the FULL NAME name field and I'm not sure either is unique! Maybe they are in combination.  When we started using QB I expected Sage like functionality and did some experiments only to find that customer updates done manually produced inconsistent outcomes.  When I thought I was updating an existing customer record sometimes a new one was generated, sometimes not.  I gave up in the end and we simply allow QB to create new customer records when we import invoices from our other systems.  Later we add things like email addresses manually - a right pain!

-Dave Sherwood