Ich habe die Lösung gefunden.
In [Desktop Entry] muss noch folgende Zeile ergänzt werden:
StartupWMClass=Portfolio Performance
Hier die englische Erklärung was es bewirkt:
" Modern Linux desktop environments (Gnome 3, Ubuntu Unity, Docky) use the WMClass property to associate windows with the owning application. Many Java-based applications have the problem where the windows created by the application are not associated with the shortcut used to start the application (so the dock contains multiple copies of the same icon).
So the field called StartupWMClass is a string that defines in which class the application is grouped in the Unity Launcher at startup."