asp.net – Paypal自适应支付返回网址是两次调用

我无法理解为什么返回网址会调用两次.

请指教.

我正在使用下面的代码.

public static ActionOutput MakeTransactionUsingPaypal(PaymentDetails payment,ShopCart shop_cart)
{
    ReceiverList receiverList = new ReceiverList();
    receiverList.receiver = new List<Receiver>();
    string action_type = "PAY_PRIMARY";
    decimal amnt_to_admin = ((shop_cart.TotalAmountToBePaid * 10) / 100);

    /*Total Amount to Admin Account */
    Receiver rec1 = new Receiver(shop_cart.TotalAmountToBePaid);
    rec1.email = Config.AdminPaypalBusinessAccount;
    rec1.primary = true;

    /*Amount after deducting to Admin Commision to Seller */
    Receiver rec2 = new Receiver(Math.Round((shop_cart.TotalAmountToBePaid - amnt_to_admin),2,MidpointRounding.ToEven));
    rec2.email = payment.PaypalEmail; // "anuj_merchant@xicom.biz";

    receiverList.receiver.Add(rec1);
    receiverList.receiver.Add(rec2);
    PayRequest req = new PayRequest(new RequestEnvelope("en_US"),action_type,Config.PaypalCancelURL,"USD",receiverList,Config.PaypalReturnURL);

    // All set. Fire the request            
    AdaptivePaymentsService service = new AdaptivePaymentsService();

    PayResponse resp = null;
    //TransactionDetail details = new TransactionDetail();

    resp = service.Pay(req);
    String PayKey = resp.payKey;
    String PaymentStatus = resp.paymentExecStatus;
    ResponseEnvelope ResponseEnvelope = resp.responseEnvelope;
    PayErrorList errorList = resp.payErrorList;
    List<ErrorData> errorData = resp.error;
    if (errorData.Count > 0)
    {
        return new ActionOutput
        {
            Status = ActionStatus.Error,Message = errorData[0].message
        };
    }
    FundingPlan defaultFundingPlan = resp.defaultFundingPlan;
    WarningDataList warningDataList = resp.warningDataList;
    string redirectUrl = null;
    if (!(resp.responseEnvelope.ack == AckCode.FAILURE) &&
        !(resp.responseEnvelope.ack == AckCode.FAILUREWITHWARNING))
    {
        redirectUrl = ConfigurationManager.AppSettings["PAYPAL_REDIRECT_URL"] + "_ap-payment&paykey=" + resp.payKey;

    }
    return new ActionOutput
    {
        Status = ActionStatus.Successfull,Message = "Redirecting to paypal...",Results = new List<string> { redirectUrl,resp.payKey }
    };
}