The DoExpressCheckoutPayment or SetExpressCheckout APIs return this error if a successful transaction has already been completed for the Express Checkout token.

Here's how to resolve this issue:

• Use the GetExpressCheckoutDetails API with the token to get transaction information, including the Transaction ID.
• Use the TransactionSearch API to find the transaction.
• If the transaction was created with an Invoice ID, use the Invoice ID as a parameter to the TransactionSearch API, or you can limit the search to a specific date and time range.

See the Developer Portal for a complete list of NVP/SOAP API error codes.