using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        List<Test> foo = new List<Test>()

        {

            new Test() { ID = "p01", Name = "Green" },

            new Test() { ID = "p02", Name = "Blue" },

            new Test() { ID = "p01", Name = "Green 2" },

            new Test() { ID = "p02", Name = "Blue 2" },

            new Test() { ID = "p03", Name = "Red" },

            new Test() { ID = "p04", Name = "Yellow" },             

        };

        foreach(var prod in foo.GroupBy(p => p.ID).Where(p => p.Count() > 1).SelectMany(x => x))

        {

            Console.WriteLine(prod.Name);

        }

    }

    public class Test

    {

        public string ID {get;set;}

        public string Name {get;set;}

    }

}