Поиск дубликатов без LINQ

public class Class1
{
public int Num; //номер по порядку
public string ID; //идентификатор
}

List<IDs> Ids = new List<IDs>() 
{ 
    new IDs { Num = 1, ID = "one" },
    new IDs { Num = 2, ID = "one" },
    new IDs { Num = 3, ID = "one" },
    new IDs { Num = 1, ID = "two" },
    new IDs { Num = 3, ID = "three" },
    new IDs { Num = 4, ID = "three" },
    new IDs { Num = 5, ID = "three" },
    new IDs { Num = 4, ID = "four" },
    new IDs { Num = 5, ID = "five" },
};
var resLinq = from id in Ids
            group id by id.ID into idGroups
            where idGroups.Count() > 1
            select new 
            {
                id = idGroups.Key, 
                values = from id in idGroups
                        select id.Num
            };
 
var resExtension = Ids
    .GroupBy(i => i.ID)
    .Where(g => g.Count() > 1)
    .Select(g => new { id = g.Key, values = g.Select(i => i.Num) });