Imagine the following 2 files:
functions.inc.php:
 	
	| <?php function TestFunction ( ) { echo 'Blah'; }
 ?>
 | 
 	
	| <?php include ( 'functions.inc.php' );
 include_once ( 'functions.inc.php' );
 
 TestFunction ( );
 ?>
 | 
(sue me for creativity 

)
Without PHPExpress you get "Blah" as expected.
If you run this example with PHPExpress:
PHP Fatal error:  Failed to load cached file (functions.inc.php)
function testfunction is already declared in test.php on line 4
This is obviously a condensed example, but some of the sites I host actually work like this, using a normal collection-file with a set of "include"'s in there and then 
include_once further along the script. Since PHP itself does not make a problem out of it, shouldn't that also be true for PHPExpress?