How can we be notify if we are encoding an invoice which was already encoded?

Here's the catch regarding the invoice numbers in QuickBooks Online, erc-acctng-gmail.

When you turn on the Custom transaction numbers feature, the Invoice no. field now be available in the Invoice page. With the said, QuickBooks will prompt you that the number was already used upon saving the invoice. From there, you can select Yes or No to confirm the action.

For more information on how the invoice number works, you can browse this link: How to change the invoice number on sales forms.

Feel free to let us know if you have additional queries regarding the invoices. Have a great day!