C＃thread和exception＆＃34; Sequence包含多个匹配元素＆＃34;

Question

此功能可更新活动用户列表。删除那些不在服务器列表中的人。添加了不在客户列表中的人。该函数是从不同的线程调用的，但为了避免混淆，我将所有内容都放在处理队列Dispatcher

中

//This function is called from different threads
        public void UpdateUsers(List<ServerUser> serverUsers)
        {
            //imitation one thread mode 
            Application.Current.Dispatcher.BeginInvoke(new Action(() =>
            {
                //remove old user
                ClientUsers.ToList().ForEach(clientUser =>
                {
                    if (serverUsers.All(serverUser => clientUser.Id != serverUser.Id))
                    {
                        OnUserEvent(clientUser, false);
                        ClientUsers.Remove(clientUser);
                    }
                });

                serverUsers.ForEach(serverUser =>
                {
                    var clientUser = ClientUsers.SingleOrDefault(clientUser => serverUser.Id == clientUser.Id);

                    //Add new user
                    if (clientUser == null)
                    {
                        clientUser = new ClientUser(serverUser);
                        ClientUsers.Add(clientUser);
                    }
                });

            }), DispatcherPriority.ApplicationIdle).Wait();//Wait completion!!!
        }

结果，我明白了 System.InvalidOperationException：Sequence在System.Linq.Enumerable.SingleOrDefault [TSource]（IEnumerable 1 source, Func 2谓词）中包含多个匹配元素

我哪里错了？