I'm not an accountant or bookkeeper, but I have been using Gnucash to enter and manage my invoices. I had an instance where I used B192116 for an invoice number and the customer paid this invoice. A month later there was some confusion and I re-entered the same invoice number B192116 for the same customer and Gnucash took it no problem. Is there a way for Gnucash to warn or stop me from entering duplicate invoices?
When I hit this problem, the solution that worked for me (on Gnucash 3.4) was to use automatic invoice numbering.
When I create a new invoice, I simply leave the invoice number blank, and Gnucash chooses the next invoice number in the sequence.
To set the current number in the sequence, choose File / Properties from the main menu. Then choose the Counters tab. Scroll down to invoices and set the current number. The next invoice you generate will have a number one greater than this.
This appears to be a pretty common problem with Gnucash without a solution (that I can find). See the following forum posts:
I recommend submitting a feature request here.