using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        int[] array = new int[] {1,2,3,4,2,4,5,7,2,2,2};

        List<int> output = new List<int>();

        Dictionary<int,bool> dict = new Dictionary<int,bool>();

        foreach(int i in array){

            if(dict.ContainsKey(i)){

                if(dict[i])

                    output.Add(i);

                dict[i] = false;

            }

            else{

                dict.Add(i,true);

            }

        }

        output.ForEach(x=>Console.WriteLine(x));

        Console.WriteLine("Hello World");

    }

}