using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

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

        var duplicates = list.GetDuplicates();

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

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

    }

}

​

public static class Extensions

{

    public static IEnumerable<TSource> GetDuplicates<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> selector, IEqualityComparer<TKey> comparer)

    {

        var hash = new HashSet<TKey>(comparer);

        return source.Where(item => !hash.Add(selector(item))).ToList();

    }

    public static IEnumerable<TSource> GetDuplicates<TSource>(this IEnumerable<TSource> source, IEqualityComparer<TSource> comparer)

    {

        return source.GetDuplicates(x => x, comparer);

    }

    public static IEnumerable<TSource> GetDuplicates<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> selector)

    {

        return source.GetDuplicates(selector, null);

    }

    public static IEnumerable<TSource> GetDuplicates<TSource>(this IEnumerable<TSource> source)

    {

        return source.GetDuplicates(x => x, null);

    }

}