Error: Error Detected:
This invoice has already been paid. For more information, please contact the merchant.
=>When we confirm the order with payment paypal . When the site goes to paypal login above error execute:
Reason for this error:
There are probably two reason:
Reason1: Configuration Setting
Reason2: Orders Overwrite
The setting of following field:
=>Do you want to specify the order number as the PayPal invoice number?
( ) True
( ) False
=> This setting should be
(* ) False
Sometimes the order id overwrite with the existing with old order..in this case you need have to delete/remove the existing order and check…..