devilya

class Program
    {
        static void Main(string[] args)
        {
            List<Order> orders = new List<Order>()
            {
                new Order()
                {
                    id = 1,
                    sku = "AR",
                    sum = 100
                },
                new Order()
                {
                    id = 2,
                    sku = "PR",
                    sum = 200
                },
                new Order()
                {
                    id = 1,
                    sku = "VR",
                    sum = 150
                }
            };

            var groupedOrders = orders.GroupBy(el => el.id)
                .Select(el => new Order()
                {
                    id = el.Key,
                    sum = el.Sum(s => s.sum),
                    sku = String.Join(String.Empty, el.Select(str => str.sku))
                });
           
        }
    }

    public class Order
    {
        public int id;
        public string sku;
        public int sum;
    }