ToolWindowPane / IVsWindowFrameNotify2.OnClose(ref uint pgrfSaveOptions) called twice

Hello there,

I have a VS2010 ToolWindowPane-deriving class that also implements IVsWindowFrameNotify2, however the .OnClose(ref uint pgrfSaveOptions) which works fine except it is being called twice every time one closes the toolwindow.

Is there any reasoning behind this? The stack traces do look slightly different for the two times but I only want to show the user a 'Unsaved Changes etc'-dialog only once & keeping track/count of closing events seems quite wrong/error-prone, too.

Does anyone have any deeper knowledge about what's going on here?

Thanks

-Jörg