MDI Childs open forms only once even with different controls

Hi, i was wonder is there was a way to write a method or some how make it so my mdi children  forms can only be shown once.  Even if the user clicks different buttons.. ie if the user opens the form from the menu strip, and tries to open the form from the menu strip again , it wont open if the child is currently open. or if he then try to open the from from the main page of the project it sill wont let the user open a new page, i want it to just pull the page to front and give it the focus. Yet if it is closed i need it to open the form again.

thanks