Web Service with nested key collecitions

I'm trying to create a web service that will expose an object with nested key collections.  When I test out the web service I get this error:

System.ArgumentException: An item with the same key has already been added.
   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Collections.ObjectModel.KeyedCollection`2.AddKey(TKey key, TItem item)
   at System.Collections.ObjectModel.KeyedCollection`2.InsertItem(Int32 index, TItem item)
   at System.Collections.ObjectModel.Collection`1.Add(T item)
   at Test.WebServices.ModelService.Test() in C:\Dev 09\Test.WebServices\ModelService.asmx.cs:line 75

I've tested just one key collection and it works great it's when I nest key collections is when it throws the errors.

This is a simplify version of the objects involve and test method I used:
    [WebMethod]
    public Class1 Test()
    {
      Class1 x1 = new Class1();

      Class2 x2 = new Class2();
      testObj y = new testObj();
      for (int i2 = 0; i2 < 5; i2++)
      {
        testObj2 y2 = new testObj2();
        for (int i3 = 0; i3 < 5; i3++)
        {
          y2.ID = i3;
          y2.Value = "test obj " + i3.ToString();
          x2.Add(y2);
        }
        y.ID = i2;
        y.Obj2 = x2;
        x1.Add(y);
      }
      return x1;
    }
  public class Class1 : KeyedCollection<int, testObj>
  {
    protected override int GetKeyForItem(testObj item)
    {
      return item.ID;
    }
  }

  public class Class2 : KeyedCollection<int, testObj2>
  {
    protected override int GetKeyForItem(testObj2 item)
    {
      return item.ID;
    }
  }

  public class testObj
  {
    private int _id;
    private Class2 _obj2;

    public int ID
    {
      get { return _id; }
      set { _id = value; }
    }

    public Class2 Obj2
    {
      get { return _obj2; }
      set { _obj2 = value; }
    }
  }

  public class testObj2
  {
    private int _id;
    private string _value;
    public int ID
    {
      get { return _id; }
      set { _id = value; }
    }

    public string Value
    {
      get { return _value; }
      set { _value = value; }
    }
  }