Quickbooks requires that display names of all customers in your Quickbooks accounts are unique.  If a validation error is thrown when an invoice is being copied from CaseFox to Quickbooks, please make sure first name and last name in Quickbooks matches first name and last name in CaseFox.  If not please make sure First Name one space Last name in CaseFox matches displayname in Quickbooks. If not, please make sure company name in casefox matches with displayname in Quickbooks.