Here's where the power of OOP (Object-oriented programming) comes in.
You can create a "template" form that contains the MainMenu with the common MenuItems. Set the scope of the MainMenu control (Modifiers property) to public; this allows you to modify it on the other forms.
Then have the other forms inherit that form. To do that, change the line:
The power of this method is that if you need to make a change to all the menus, you only need to change the template form then rebuild and all the inherited forms will be changes as well.