private static object asynclocker = new object();
        private static List<string> processingOrders = new List<string>();

 private static bool CheckSession(string orderid)
        {
            lock (asynclocker)
            {
                bool isExists =  processingOrders.Find(s => s.ToLower().Trim() == orderid.ToLower().Trim()) == null;
                if (isExists)
                {
                    processingOrders.Add(orderid.ToLower().Trim());
                }

                return isExists;
            }
        }


使用：
//正在处理中
                    if (!CheckSession(notifyModel.OutTradeId))
                    {
                        throw new JSJException("订单正在处理中……");
                    }

 List<Task<string>> tasks = new List<Task<string>>();

            for (var i = 0; i<=5; i++)
            {
                tasks.Add(Task.Factory.StartNew<string>(() =>
                {
                    return this.PayOrder();
                }));
            }

            Task.WaitAll(tasks.ToArray());

            foreach (var task in tasks)
            {
                Console.WriteLine(task.Result);
            }