my website is http://www.antinuke.acaonline.biz the problem im haveing is that at the top of the page there is a messages that says "Warning: ob_start(): output handler 'ob_gzhandler' cannot be used twice in /home/acaonli/public_html/nuke/includes/page_header.php on line 59" what does it mean? How can I fix it?

Thanks
Kryptonic