Paypal says "This invoice has already been paid."

I'm sure this is my fault somehow, but what would cause this?

"This invoice has already been paid. For more information, please contact the merchant."

This is from Paypal...NO idea what cause this or how to fix it.

Here is the documentation related to that problem:
www.hikashop.com/en/support/documentatio...l-error.html#already

Ah... I wonder if this could be caused by the fact that I completely busted my site and had to rebuild it from scratch, so the order numbers started all over, but I used the same Paypal account.

Yes, that's indeed the problem.

"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".
(This is now called "My selling tools" and "Block payments")
* Under "Block Accidental Payments", choose "No, allow multiple payments per invoice ID".