If you try to sync an invoice and come across the following error:
QuickBooks response: Duplicate invoice number. You must specify a different number. This number has already been used.
This is Quickbooks indicating that the invoice number used on the invoice in Knowify matches a number previously used by an invoice or credit memo in Quickbooks.
In cases like these we would recommend editing the invoice number to something unique that hasn't been used yet, and then pushing the invoice to Quickbooks again. You can find more information on how to change an invoice number in Knowify here.
You can also set the last invoice number used in Knowify, so that we increment from this number each time, reducing the possibility of landing on a number that is already used in Quickbooks.