Hi JenoP, 

This isn't a browser issue. We are having the same issue, whereby new invoices are being assigned invoice numbers that have already been used. Customer Transaction Number is turned off. It can't be a browser issue because our invoices are generated through Quickbooks API which automatically assigns the invoice number upon creation. 

This issue only seems to come up within the past 3-4 months. I have read about a number of other users having the same issue recently. Could there have been a change that was made in the backend that could have contributed to this issue?