using System;

using System.Linq;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var arr = new[] {

            new Obj{

                key = 1,

                val = "Um"

            },

            new Obj{

                key = 2,

                val = "Dois"

            },

            new Obj{

                key = 2,

                val = "DoisDiNovu"

            }

        };

        var lkp = arr.ToLookup(x=>x.key);

        Console.WriteLine(JsonConvert.SerializeObject(lkp[1]));

        Console.WriteLine(JsonConvert.SerializeObject(lkp[2]));

        Console.WriteLine(JsonConvert.SerializeObject(lkp[3]));

    }

    public class Obj{

        public int key;

        public string val;

    }

}