using System;

using System.Collections.Generic;

​

using System.Linq;

public class Program

{

    static List<Tuple<string, string>> myList = new List<Tuple<string, string>>()

    {

        Tuple.Create<string, string>("A", "B"),

        Tuple.Create<string, string>("B", "A"), // duplicate

        Tuple.Create<string, string>("C", "B"),

        Tuple.Create<string, string>("C", "B"), // duplicate

        Tuple.Create<string, string>("A", "D")              

    };

    public static void Main()

    {

        myList

            .Select(x => new[] { x.Item1, x.Item2 }.AsEnumerable().OrderBy(s => s).ToArray())

            .Select(x => Tuple.Create<string,string>(x[0], x[1]))

            .Distinct()

            .Dump();

    }

}