Hi, 

Thank you for responding back so fast!

I completely forgot that the issue isn't duplicate invoice numbers, it's duplicate invoices with different invoice numbers on different days. It's only happening to two customers, who are related - separate accounts but we have to change invoices from one customer to the other because they were billed to the wrong entity. 

So could that have something to do with QBs desktop changing the customer from one to another because these two customers are the only ones we have to fix. 

Below are two invoices, one is voided because it was a duplicate of another invoice from another day. 

Thanks again for your help!

Kelly