Paypal: This invoice has already been paid.

Earlier, I was talking about transferring an OSCommerce store that uses MySQL. Today I was told that the test orders being placed on the store were receiving an error that "This invoice has already been paid."

At first I thought that it may be a bug as the following post suggests:
Paypal: This invoice has already been paid.

But then after a little bit of thinking I realized that this problem was ocurring due to a discrepancy in order numbers. The client had asked me to transfer the store and let them test it for a while before they make the switch. During this testing phase, the "live implementation" of the store was already receiving orders. Therefore when the client tried to place an order using the "test implementation" they received the errors that "this invoice has already been paid".

I told the client that once they are ready for a switchover, I will resynch the databases and the problem should be gone.

Frank