It is possible for PayPal to report an order as already being paid if they have received an identical invoice number in the past. This usually occurs if the merchant is using the same PayPal account on multiple stores, and eventually a previously issued invoice number sequence is sent from the second store.
There are a two separate options you can use to resolve this issue:
In Your PayPal Account:
- Login to your PayPal account by going to www.PayPal.com.
- Click "Profile" on the top menu bar.
- Under "Selling Preferences", click "Payment Receiving Preferences".
- Under "Block Accidental Payments", choose "No, allow multiple payments per invoice ID".
In Your Shift4Shop Account:
- Log into your Shift4Shop Online Store Manager
- Go to Settings >General>Store Settings
- Click on the "Checkout" tab and look for the Invoice Number portion of the page to click on "Edit Invoice Number"
- Increase the invoice number so that it surpasses the last invoice number received by PayPal, and click "Save Changes".
Either of these methods will fix the error going forward, you will not need to do both.