using System;

using System.Collections.Generic;   

using System.Linq;

​

public class Program

{

    public static void Main()

    {

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

        var distincts = list.Distinct();

        var duplicates = distincts.Where(e => list.Where(p => p == e).Count() > 1);

        Console.WriteLine(string.Join(",", duplicates));

    }

}