what's the matter with wxPython(Version 2.4.0), It doesn't work crrectly

[Baby]

I wrote a test program in wxPython.(The GUI Development  is Boa
constructor ,
version 0.20).I placed a wxButton on a wxFrame, and I produced a 
wxEVT_COMMAND_BUTTON_CLICKED event . Just like this:

      def OnButton1Button(self, event):        
        dlg = wxMessageDialog(self, 'Message',
          'Caption', wxOK | wxICON_INFORMATION)
        try:
            dlg.ShowModal()
        finally:
            dlg.Destroy()        
        event.Skip()
   but when I click the button , the function is performed two times.
I don't know what's matter with it. Who can help me, Thanks a lot.
Sorry my poor English