using System;

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>();

        list.Add(new smth { id = 1 });

        list.Add(new smth { id = 1 });

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

        IEnumerable<smth> smths = groups.Select(x => x.FirstOrDefault());

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

        foreach (var s in newList)

        {

            Console.WriteLine(s.id);

        }

    }

}