using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var C1= new Combination{Id=1, CombOne=1, CombTwo=2};

        var C2= new Combination{Id=2, CombOne=2, CombTwo=1};

        var C3= new Combination{Id=3, CombOne=1, CombTwo=2};

        var C4= new Combination{Id=4, CombOne=3, CombTwo=3};

        var C5= new Combination{Id=5, CombOne=2, CombTwo=1};

        var list = new List<Combination> { C1, C2, C3, C4, C5 };

        var set = new HashSet<Combination>(new CombinationComparer());

        var invalid = list.Aggregate(new List<Combination>(list.Count),

                              (a,c) => 

                              {

                                 if (!set.Add(c))

                                 {

                                     a.Add(c);

                                 }

                                 return a;

                              });

        Console.WriteLine(string.Join( ",", invalid.Select(c => c.Id)));

    }

}

​

public class Combination

{

    public int Id { get; set; }

    public int CombOne { get; set; }

    public int CombTwo { get; set; }

}

​

public class CombinationComparer : IEqualityComparer<Combination>

{

     public bool Equals(Combination c1, Combination c2)

     {

         if (ReferenceEquals(c1,c2))

         {

              return true;

         }

​

         if (c1 == null || c2 == null)

         {

              return false;

         }

​

         return (c1.CombOne == c2.CombOne && c1.CombTwo == c2.CombTwo)

                    || (c1.CombOne == c2.CombTwo && c1.CombTwo == c2.CombOne);

    }

    public int GetHashCode(Combination c)

    {

        if (c == null)

        {

            return 0;

        }

        unchecked

        {

            // it's important that this be commutative so we don't

            // do the multiply by a prime to differentiate

            // them.

            return c.CombOne.GetHashCode() + c.CombTwo.GetHashCode();

        }

    }

}