I have some insight that can help you handle this duplicate invoice right away. The only way to completely null the income generated by an invoice is to Void or Delete it. By voiding, the transaction remains in your books for later reference, whereas deleting will remove it completely. These steps will help you do this:
Void or Delete an Invoice
From QuickBooks Online, navigate to the Sales tab and Customer section.
Find the duplicate invoice on the list, using the Filter menu to narrow the list if necessary. Click the transaction to open it.
From the More menu at the bottom of the Invoice screen, select Void or Delete.
Please note: if a Payment transaction is assigned to one of the invoices, be sure to delete the opposite so as not to alter Accounts Receivable. We offer an article further detailing the steps necessary to void/delete an invoice in QuickBooks Online if you're interested. I'm also including a brief video example in which I void a duplicate invoice:
The instructions here will help you tackle that duplicate invoice without issue. Most importantly, be sure to consult with an accounting professional to determine if voiding/deleting the invoice is the best course of action. Intuit's Find-An-Account service can put you in touch with a local, certified expert to take a look at your specific situation.
Please touch base with me here if there's anything else you need, the Community always has your back. Thanks for reaching out, wishing you and your business the best.