Smushit pro will not activate - gives a fatal error

Hi - when I try to activate Smush Pro - I get this:
Fatal error: Cannot redeclare get_plugin_dir() (previously declared in /.../wp-content/plugins/wp-smushit/wp-smush.php:118) in /.../wp-content/plugins/wp-smush-pro/wp-smush.php on line 116

Any ideas?