using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var list = new List<Order> {

            new Order { Id = 1, IsComplete = true },

            new Order { Id = 2, IsComplete = false },

            new Order { Id = 2, IsComplete = true },

            new Order { Id = 3, IsComplete = true },

            new Order { Id = 4, IsComplete = true },

            //new Order { Id = 4, IsComplete = false },

            new Order { Id = 5, IsComplete = false }

        };

        // Select will return distinct values of Key property.

        // SelectMany will return all duplicates objects.

        var duplicates = list.GroupBy(s => s.Id)

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

                             .Select(g => g.Key); // or .SelectMany(g => g)

        if (duplicates.Count() > 0)

        {

            foreach (var dupe in duplicates)

            {

                Console.WriteLine(dupe);

            }

            Console.WriteLine("Duplicates: {0}", string.Join(", ", duplicates.ToArray()));

        }

    }

    public class Order

    {

        public int Id { get; set; }

        public bool IsComplete { get; set; }

        public override string ToString()

        {

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

        }

    }

}