New invoice is being assigned an old/used number

Allow me to help share some information about how the invoice number works in QuickBooks Online, cmlasellers.

QuickBooks will use the same invoice number if the other transaction is linked to a project. To get this sorted out, you can edit the invoice and enter a new number.

Here's how:

1. Click Sales in the left panel and select All Sales.
2. From the Filter drop-down, choose Invoices as the Type of transaction.
3. Tap Apply.
   
4. Sort the No. column. It toggles the numbers in ascending and descending order.
5. Check for duplicates.
   
6. Open the invoice and change the number from there.
7. Hit Save and close.

To know other ways on how to look for duplicate transactions, you can go through the articles below. It also gives you steps on how to search by customer and vendor names.

How to find duplicate transaction numbers.

Search for transactions.

This should keep you moving today, cmlasellers. Don't hesitate to let me know if I can be of additional assistance while working in QuickBooks. I'm always here to keep helping. Have a good one!