You could declare and assign the instance to a private varibale in your form and simply call the Show method in your button click event if you only want one instance.
One other method would be to keep a boolean variable in your form and set it to true when you open a form, and in your event check if false and create new form, else do not create a new form.
Yet another method would be to enumerate all the MDI children and see if any form in there is of the same type or with the same name:
For Each f As Form In Me.MDIChildren
If TypeOf f Is YourFormType Then
'form of that type already open
End If
Next
Simply saying, there is many possibilities :).