using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main(string[] args)

    {

        List<Item> items = new List<Item>

        {

            new Item { Id = 1, Value = 12},

                new Item { Id = 1, Value = 900 },

                new Item { Id = 1231, Value = 0 },

                new Item { Id = 1, Value = 577 }

        };

        List<Item> nonDupes = items.GroupBy(x => x.Id).Select(x => x.First()).ToList();

        List<Item> dupes = items.Except(nonDupes).ToList();

        Console.WriteLine("non-dupes: [[{0}]]", string.Join("], [", nonDupes));

        Console.WriteLine("dupes: [[{0}]]", string.Join("], [", dupes));

    }

}

​

public class Item

{

    public int Id { get; set; }

    public int Value { get; set; }

    public override string ToString()

    {

        return string.Format("Id: {0}, Value: {1}", Id, Value);

    }

}