Each order the advertiser makes on the system can only be paid once
This is to prevent your advertisers to accidentally pay for the same pixels twice or more.
A user who received this error already paid for this order and there is no need to pay again. (The 'invoice' variable issued for each order can only be used once per buyer)
If you are testing the script using a PayPal sandbox account, then you are getting this error because you ran a test before where you were using the same invoice id. (For example, this can happen if you make an order, clear your database / re-install, and then make a new test order) OR another script that you were testing had an exactly the same invoice id & item id. To get around this issue, start a new sandbox account for testing the script each time.
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