How do I prevent duplicate Sales Order or Invoice numbers? There is a feature available on the backend that enforces unique Sales Orders and/or Invoices numbers. Please contact us today to enable this feature. Related articles How do I change my Invoice number sequence? How does Sales Order/Invoice numbering work in QuickBooks Commerce? How do I change my Sales Order number sequence? What are the various color codes for stock level quantity? Xero Error: There is already a xxx invoice in your account.