using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    class smth

    {

        public int id;

    }

​

    public static void Main()

    {

        List<smth> list = new List<smth>();

        IEnumerable<IGrouping<int, smth>> groups = list.GroupBy(x => x.id);

        IEnumerable<smth> smths = groups.SelectMany(group => group);

        List<smth> newList = smths.ToList();

    }

}