[d]wxPython的button点击响应事件为何发生两次

import wx

class MyFrame(wx.Frame):
    def __init__(self):
        wx.Frame.__init__(self, None, title="Hello My World")
        b = wx.Button(self, -1, "Create and Show a Frame", (50,50))
        self.Bind(wx.EVT_BUTTON, self.OnButton, b)
        self.Bind(wx.EVT_CLOSE, self.OnClose)
        self.Show()
    def OnButton(self, evt):
        dlg = wx.MessageDialog(self, 'Hello from Python and wxPython!',
                                'A Message Box',
                                wx.OK | wx.ICON_INFORMATION
                                #wx.YES_NO | wx.NO_DEFAULT | wx.CANCEL | wx.ICON_INFORMATION
                                )
        dlg.ShowModal()
        dlg.Destroy()
        evt.Skip()
    def OnClose(self, evt):
        dlg = wx.MessageDialog(self, 'Are you sure you want to close My World?',
                                'Closing...', wx.YES_NO | wx.ICON_QUESTION)
        ret = dlg.ShowModal()
        dlg.Destroy()
        if ret == wx.ID_YES:
            evt.Skip()

app = wx.App(0)
MyFrame()
app.MainLoop()