Use require_once in more places

I got:
PHP Fatal error:  Cannot redeclare file_notice() (previously declared in /etc/inc/notices.inc:55) in /etc/inc/notices.inc on line 91
So there are places that notices.inc gets require() when already required.
I was running 2.1.3 in a Virtualbox VM and trying to do an upgrade to 2.2 using pfSense-Full-Update-2.2-DEVELOPMENT-i386-20140508-1205.tgz - I guess the system was trying to tell me something that went wrong, but instead it ran into trouble with require of notices,inc while trying to tell me something else.

For *.inc that declare included functions, is there ever a reason not to use require_once?
i.e. should many (or all?) require() statements actually be changed to require_once() to make sure to avoid any issues with multiple/nested/... require() statements?

Note: I believe this could (should) also be applied to 2.1-branch, since it happened to me on a 2.1.3 system (that I was attempting to update...)