Error with class_mail.php when posting

Get this message everytime i post. Was fine up until now

If i refresh it says the post is a duplicate and it redirects me so its actually posting the first time but not redirecting properly

Fatal error: Call to undefined function addDebugMessage() in /home/blackbum25/public_html/blackhat-seo/includes/class_mail.php on line 863

same here, if you refresh you will get double posting warning