An invoice payment shows up twice when I view the Transaction Report for Sales on my P

I can clear things out for you, @rich44.

The possible reason why your invoice payment doubled is that you manually received it instead of matching it to the invoice. 

Please know that when you accept credit card payments in QuickBooks Online, it'll automatically deposit the transaction once your customer paid their invoices. That said, you'll have to match it to the invoice from the Banking page to avoid duplicate entries. To get rid of this, you'll want to delete the payment that was received manually. Then, match the deposited payment from the Banking page.

Here's how to delete the manually received payment:

1. Go to the Accounting menu and click Chart of Accounts.
2. Locate and click the View register of your bank.
3. Find and select the manually received payment and click Delete.
4. Hit OK to confirm.

After deleting it, let's match the invoice to the deposited payment from the Banking page.

1. Click Banking from the left menu.
2. Select the appropriate bank account where the payment is deposited.
3. Click the deposit and select Find a match.
4. Filter the Date range that includes the date when the invoice payment was created.
5. Check the Payment and then click Save.

I'm adding this article to learn more about matching transactions: Categorize and match online bank transactions in QuickBooks Online.

You might also want to read this article for more information on how QuickBooks process payments: Common questions about payments deposits in QuickBooks Online.

Keep in touch if you need any more assistance with this, or there's something else I can do for you. I've got your back. Have a good day.