List<User> list = new List<User> { 
            new User{Id=1,Num=10},
            new User{Id=1,Num=20},
            new User{Id=2,Num=50},
            };

            var query = from l in list
                        group l by l.Id into g
                        select new { 
                            g.Key,
                            Num = g.Sum(l=>l.Num)
                        };
            foreach (var item in query)
            {
                Console.WriteLine(item.Key + ":" + item.Num);
            }