using System;

using System.Collections.Generic;

using System.Linq;

​

​

public class ObjectWithSize

{

    public int Size {get; set;}

    public ObjectWithSize(int size)

    {

        Size = size;

    }

}

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine("start");

        var list = new List<ObjectWithSize>();

        list.Add(new ObjectWithSize(12));

        list.Add(new ObjectWithSize(13));

        list.Add(new ObjectWithSize(14));

        list.Add(new ObjectWithSize(14));

        list.Add(new ObjectWithSize(18));

        list.Add(new ObjectWithSize(15));

        list.Add(new ObjectWithSize(15));

        var duplicates = list.GroupBy(x=>x.Size)

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

        foreach (var dup in duplicates)

            foreach (var objWithSize in dup)

                Console.WriteLine(objWithSize.Size);

    }

}