When changed, PayPal accepts payments with no error messages, even for invoices with duplicate IDs.
To resolve the problem, allow multiple payments per invoice ID in PayPal’s Payment Receiving Preferences. The issue occurs when invoices with the same IDs are sent to PayPal several times. Payment has already been made for this InvoiceID (#10412: Duplicate invoice)
When submitting payment, Customers may see an error for a duplicate invoice: This article provides a fix for the PayPal gateway rejected request - duplicate invoice issue. PayPal gateway rejected request - duplicate invoice issue
