PHP error: Cannot modify header information – headers already sent

Possible Duplicate:
Headers already sent by PHP

So I have this output on my page.. not understanding why I have it popping up. I'm new to php though, so maybe it's something easy to fix

-I have a header.php file, which includes all important info, as well has the banner of the page. This header.php is included on every page.

-I have it checking the session value to make sure user is allowed to be at a certain page. If user is not allowed to be there, I kick them back to login page

This is where the error comes up though. This is what I have:

include_once ("header.php");

if ($_SESSION['uid']!='programmer')
{                        

header('Location: index.php');
echo 'you cannot be here';
exit;
}   

The index that it is redirecting to also has the header. So is having these multiple header references giving me this error? I see no other way to do this, and it's driving me nuts!