include_once() then what?

Hi, I am a bit confused about include_once(), although it is pretty obvious that it will include anything only once.. I have been going through the manual and I see the defination/use of include_once but one thing the manual does not mention is what happens if you accidentally include_once in the same file twice by accident? on my damn hosting webserver errors are off or I would have tested it myself but do any of you guys know? Does it throw a warning? error? what? I think I really should get myself a local server etc installed on my machine, I tried phpTriad and was not all that pleased and for some reason FoxServ didnt work on my machine so i uninstalled it...anything else thats really good out there? After visiting hotscripts I was thinking of getting Phperl (http://www.hotscripts.com/Detailed/18015.html) any comments? Cheers, -Ryan