how do I do a credit note as a duplicate invoice was done and send

Welcome to the Community, Ray.

You can safely delete the duplicate invoice you created, as no payment that has been issued yet. 

Given that a credit note is primarily used to provide a refund or adjust the balance on a customer's account when an overpayment has occurred on their invoice. Thus, deleting your invoice will resolve the concern.

You can follow the steps below:

1. Go to Sales, and select Invoices from the menu.
2. Use the Date filter to locate the duplicate invoice.
3. In the Action column, select the dropdown arrow, then click Void or Delete.

I'll also share this article in case you still like to learn more about applying a credit note in QuickBooks Online (QBO): Handle customer refunds in QBO.

Feel free to reply to this post if you need further assistance.