In your PayPal account, go to "Profile" - > "Payment Receiving Preferences". In there you will have the option:
Block accidental payments:
You may prevent accidental payments by blocking duplicate invoice IDs
Yes, block multiple payments per invoice ID
No, allow multiple payments per invoice ID
PayPal will now allow all orders to be paid, even if the order ID has already been paid previously. A potential problem is if customers pay an invoice twice by mistake (e.g. refreshing browser, hitting button twice etc.).