PayPal - Allowing Duplicate Invoice IDs
In the case where you may need to make an adjustment on a payment / invoice that you have already submitted through PayPal, you may get an error because the Invoice ID already exists.
- Login to your Business PalPal account (https://www.paypal.com/businessmanage/preferences/payments)
- Go to the Payments Preferences section
- Under the "Block accidental payments:" section, select the option "No, allow multiple payments per invoice ID"