Error: Error Detected:
This invoice has already been paid. For more information, please contact the merchant.

Error Time:
=>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

For Reason1:
=>In Admin/Module/Payment/
The setting of following field:
Synchronize Invoice:
=>Do you want to specify the order number as the PayPal invoice number?
( ) True
( ) False
=> This setting should be
(* ) False

For Reason2:
Sometimes the order id overwrite with the existing with old this case you need have to delete/remove the existing order and check…..