after discussing the issue extensively with sebastien bacher i agree that its not a bug, on upgrades the update-manager will care for the switch and remove xscreensaver, on new installs only gnome-screensaver will be installed. if you manually install a second screensaver this behavior is right to enable you to configure both.
** Changed in: gnome-screensaver (Ubuntu) Status: Confirmed => Rejected -- Screensavers option shown twice in System-Preferences menu https://launchpad.net/bugs/32385 -- desktop-bugs mailing list desktop-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/desktop-bugs