avoid to eventually try to init Gui twice

src/Main/FreeCADGuiPy.cpp

@@ -326,7 +326,10 @@ PyMOD_INIT_FUNC(FreeCADGui)
         App::Application::Config()["AppIcon"] = "freecad";
         App::Application::Config()["SplashScreen"] = "freecadsplash";
         App::Application::Config()["CopyrightInfo"] = "\xc2\xa9 Juergen Riegel, Werner Mayer, Yorik van Havre 2001-2017\n";
-        Gui::Application::initApplication();
+        // it's possible that the GUI is already initialized when the Gui version of the executable
+        // is started in command mode
+        if (Base::Type::fromName("Gui::BaseView").isBad())
+            Gui::Application::initApplication();
 #if PY_MAJOR_VERSION >= 3
         static struct PyModuleDef FreeCADGuiModuleDef = {
             PyModuleDef_HEAD_INIT,