newbie and ask show window

["Ade Amrillah" <adegaos gmail com>]

hi all
i'm newbie in gtk+. i was create simple programme for testing "gtk_widget_show()" function.
my program has tow windows
first window i put button for call second window, i develop GUI with Glade and anjuta for code.
my code are
 
void on_openbtn_clicked (GtkButton  *button,gpointer user_data)
{
     GtkWidget * window2;
    window2=create_window2();
    gtk_widget_show(window2);
   
}

and it's working. but if i press button again  a new window (window2) will be create.
i want just one window created if i press again button. 
ask:  how i change the programme for show window for only one show?
for   illustrated:
i use gedit and i click preferences (menu edit->preperences) just one window showed  and if i click again did not show again new window.  sorry for my english :)

thx
amrillah