using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var a = new List<int> { 1, 1, 2, 3, 3, 4, 4, 5 };

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

        foreach(int b in a)

        {

            var x =a.Where(q=>q ==b);

        if(x.Count()>1)

        {

            if(!d.Contains(b))

            d.Add(b);

        }

        }

        foreach (var e in d)

        {

        Console.WriteLine(e);

        }

    }

}