A successful transaction has already been completed for this token.

Hi,
having just upgraded the site to 1.5.1, PayPal express check out appears to be trying to take multiple payments.

Request Parameters: {DoExpressCheckoutPayment}
Array
(
[AMT] => 50.68
[ITEMAMT] => 42.00
[SHIPPINGAMT] => 8.68
[L_NUMBER0] => CMC A004
[L_NAME0] => 1:18 / 1:24 Collector's Showcase [1054]
[L_QTY0] => 1
[L_AMT0] => 42.00
[ALLOWEDPAYMENTMETHOD] => InstantPaymentOnly
[SHIPTONAME] => xxxxxxxx
[SHIPTOSTREET] => xxxxx
[SHIPTOSTREET2] => xxxxx
[SHIPTOCITY] => xxxxx
[SHIPTOSTATE] => xxxxx
[SHIPTOZIP] => xxxxx
[SHIPTOCOUNTRYCODE] => GB
[BUTTONSOURCE] => ZenCart-EC_us
[RETURNFMFDETAILS] => 0
[CUSTOM] => EC-1049-1382655521
[INVNUM] => 1049-1382655521-[AuthenticModelscouk]
[TOKEN] => EC-4HP065293V255523V
[PAYERID] => FKT2FRUWALDVQ
[PAYMENTACTION] => Sale
[NOTIFYURL] => http://authenticmodels.co.uk/ipn_main_handler.php
[METHOD] => DoExpressCheckoutPayment
[CURRENCYCODE] => GBP
[USER] => xxxxxx.com
[PWD] => ****************
[VERSION] => 61.0
[SIGNATURE] => ****************************************************qsOh
)

Response:
Array
(
[TIMESTAMP] => 2013-10-24T22:58:42Z
[CORRELATIONID] => c698509a968e3
[ACK] => Failure
[VERSION] => 61.0
[BUILD] => 8252112
[L_ERRORCODE0] => 10415
[L_SHORTMESSAGE0] => Transaction refused because of an invalid argument.
See additional error messages for details.
[L_LONGMESSAGE0] => A successful transaction has already been completed
for this token.
[L_SEVERITYCODE0] => Error
[CURL_ERRORS] =>
)

Any suggestions please?

Thanks

Are there any informative PayPal debug files in your log directory?

Try this thread:

http://www.zen-cart.com/showthread.php?179524-10415-PayPal-Express-error-code&highlight=paypal+10415