using System;

using System.Collections.Generic;

using System.Data;

using System.Data.SqlClient;

using System.Linq;

using System.Net.Sockets;

using System.Text;

using System.Threading.Tasks;

​

public class Program

{

    public static void Main()

    {

        Console.Write("type a sentence: ");

String str = "this is my house";

​

​

var result = str.Replace(" ", "")

     .GroupBy(_ => _)

     .Where(x => x.Count() > 1)

     .Select(x => x.Key);

​

​

      Console.WriteLine("number of duplicates: " + result.Count() +"\r"+"duplicates are: " + string.Join(" ",result));

    }

}