[Plugin: PHPBB Single Sign On] Fatal error

After installing the plugin the sing-in is not working both in wp and phpbb. The browser shows a message as follows:

Fatal error: Cannot redeclare validate_email() (previously declared in /usr/home/acadwars/public_html/academywars.com/wp-includes/ms-deprecated.php:136) in /usr/home/acadwars/public_html/academywars.com/forum/includes/functions_user.php on line 1748

Do anyone know its solution.

http://wordpress.org/extend/plugins/phpbb-single-sign-on/