Smush Pro Activation Fatal Error

I am getting this error when trying to activate Smush Pro:

Fatal error: Cannot redeclare get_plugin_dir() (previously declared in /home/mammaroc/public_html/wp-content/plugins/wp-smushit/wp-smush.php:118) in /home/mammaroc/public_html/wp-content/plugins/wp-smush-pro/wp-smush.php on line 116