运行WPF MVVM应用程序时，显示两个主窗口实例

I develop WPF MVVM application in MS VS 2015 Professional (.NET Framework 4.6). When I run my WPF MVVM application then two instances of my application main window are displaying. Why does it have place? Below is markup of my application main window - MainWindow.xaml:I develop WPF MVVM application in MS VS 2015 Pr