Great!!! I discovered the error,
When you copy a form (using VStudio 2005) and get “Copy of MyForm” often you get the error “Duplicate items are not supported by the "Resources" parameter”
Change the name of the new and old form. When you changed the new form’s name VS renamed the old form too. But, what happens? If you rename the old form, the new form gets the same name….. Just change the name at level code, rename de Class name on both files: Class file and Designer file of the partial class
Thanks to Catherine Eibner
I found the clue to solution on her blog..