showing a form that lies in bakground

Hi,

I have created a Shared Add-in it contains a button, when you click on it, a 
System.Winidwos.Forms.Form shows up, now i have two problems that i can not 
solve them,

1. every time  you click on the button it shows a new form, I want that it 
shows just one form not more, even if you clicked on button many times. I 
mean I don't want to disable the button , the button must be enabled always, 
because of the following point:

2. when you work on the office document the form lies in bakground, when you 
clik on the button the form must shows up agin . how?

thanks you all