how can i checked if form is already open

Question

hello friends,

i want to know that if i have already open form then same form will not open again... is it possible in Windows Application using C#...
and i m using MDI form and other then is Child form.. i m given you code and i dont knowhow can i solve my problem...]

//code for open form
 Parties_Entry parties = new Parties_Entry();
  parties.MdiParent = Main_Form.mainObj;
   parties.Show();           
Reply me ASAP...