using System;
using System.Collections.Generic;
using System.Linq;


namespace myQuestion
{
    class Program
    {
        public class DuplicateData
        {
            public string address { get; set; }
        }

        public class Com : IEqualityComparer<DuplicateData>
        {
            public bool Equals(DuplicateData x, DuplicateData y)
            {
                return x.address.Equals(y.address);
            }

            public int GetHashCode(DuplicateData obj)
            {
                return obj.address.GetHashCode();

            }
        }

        static void Main()
        {
            List<string> lst = new List<string>();
            lst.Add("a");
            lst.Add("b");
            lst.Add("c");
            lst.Add("p");

            List<DuplicateData> list = new List<DuplicateData>()
    {
        new DuplicateData{address="a"},
        new DuplicateData{address="a"},
        new DuplicateData{address="a"},
        new DuplicateData{address="b"},
        new DuplicateData{address="b"},
        new DuplicateData{address="c"},
        new DuplicateData{address="d"},
        new DuplicateData{address="e"},
        new DuplicateData{address="f"},
    };

            var dup = list.Distinct(new Com());
            var RightJoin = from x in dup
                            join y in lst
                            on x.address equals y
                            into right
                            from z in right.DefaultIfEmpty(x.address)
                            select new
                            {
                                UniqueAddress = z,

                            };

            foreach (var ex in RightJoin)
            {
                System.Console.WriteLine(ex.UniqueAddress);
            }
            System.Console.ReadLine();
        }
}
}