using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        //************** First Way In Linq **************

        int[] numbers = { 1, 2, 2, 2, 3, 3, 2 };

​

        var duplicates = numbers.GroupBy(n => n)

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

                                .Select(g => g.Key).ToArray();  

​

        Console.WriteLine("Duplicate numbers: " + string.Join(", ", duplicates));

        //************** Second Way  Manually **************

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

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

​

        foreach (var value in numbers)

        {

            dict.TryGetValue(value, out int count);

            dict[value] = count + 1;

        }

​

        foreach (var d in dict)

        {

            if (d.Value > 1)

            {

                dRocrd.Add(d.Key);

            }

        }

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

    }

}