using System;

using System.Collections.Generic;

using System.Linq;          

​

public class Program

{

    public static void Print(List<Artifact> list)

    {

        foreach(var a in list)

        {

            Console.WriteLine(a.Id+" "+a.Type+" "+a.F);

        }

    }

    public static void Main()

    {

        List<Artifact> artifacts = new List<Artifact>();

        Artifact a1 = new Artifact{Id = 1, Type = "a", F = "koko"};

        Artifact a2 = new Artifact{Id = 1, Type = "a", F = "dup"};

        Artifact a3 = new Artifact{Id = 1, Type = "b", F = "koko"};

        Artifact a4 = new Artifact{Id = 2, Type = "b", F = "koko"};

        Artifact a5 = new Artifact{Id = 2, Type = "b", F = "dup"};

        Artifact a6 = new Artifact{Id = 3, Type = "c", F = "koko"};

        Artifact a7 = a1;

        artifacts.Add(a1);

        artifacts.Add(a2);

        artifacts.Add(a3);

        artifacts.Add(a4);

        artifacts.Add(a5);

        artifacts.Add(a6);

        artifacts.Add(a7);

        Print(artifacts.GroupBy(artifact => new { artifact.Id, artifact.Type}).Select(g => g.First()).ToList());

        Console.WriteLine("========================================");

        Print(artifacts.GroupBy(artifact => new { artifact.Id, artifact.Type}).Where(gr => gr.Count() > 1).Select(g => g.ElementAt(1)).ToList());

    }

}

​

public class Artifact

{

    public int Id { get; set; }

    public string Type { get; set; }

    public string F {get; set;}

}

​

​