To reproduce:
- rename xnview.ini file
- change language to English, if needed
- in options > general > mode when starting with a file > select "fullscreen - edit"
- open any image file in explorer
= menu is show in fullscreen
- press a middle mouse button to go to viewer
= xnview icon and the system buttons are shown twice


If you can't reproduce, try to change the language by Alt+F12 first, then repeat the steps above.
XnView 1.91.6 (Oct 19 2007) - complete zip version, Windows XP SP2.