using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        IEnumerable<int> ids = new List<int>() { 1, 2, 3};

             var query = ids.GroupBy(x => x)

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

              .SelectMany(z => z);

        Console.WriteLine(query);

    }

}