PHP error: Cannot modify header information – headers already sent

Possible Duplicate:
href="https://stackoverflow.com/questions/8028957/headers-already-sent-by-php">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!