*rilegge il proprio post*

Documentandomi sul sito ufficiale di php.net pare che il rapporto sia differente tra i due comandi:

Per Include

The include() statement includes and evaluates the specified file.

http://it.php.net/manual/en/function.include.php

Per Include_once

The include_once() statement includes and evaluates the specified file during the execution of the script. This is a behavior similar to the include() statement, with the only difference being that if the code from a file has already been included, it will not be included again. As the name suggests, it will be included just once.

include_once() may be used in cases where the same file might be included and evaluated more than once during a particular execution of a script, so in this case it may help avoid problems such as function redefinitions, variable value reassignments, etc.

http://it.php.net/manual/en/function.include-once.php

Personalmente mi ha chiarito la differenza dei due comandi ed il loro relativo utilizzo.