I try and open a new invoice and it uses an already used invoice number. Any ideas to why or how to fix

It's my pleasure to help you on fixing your invoices, bill181.

The prompt you're getting will occur in QBO when two or more invoices have the same number.

To fix this, you can search for the transaction using the invoice number at issue to check if an invoice transaction exists. If you find it, you can perform the necessary actions of editing them or using another number for the invoice you will create.

Here's how:

1. Select the magnifying glass icon beside the bell icon.
2. Enter the invoice number.

To record the payment you receive from your customers, check out this article: Record invoice payments in QBO.

Feel free to add a reply below if you have additional concerns about invoices. We'll get back to you as soon as possible. Stay safe!

@MelroseV  You didn't answer the user's question. Did you even read the question?