WPMU Dev Dashboard

Every time I install the plug-in to wordpress the site throws out the following error: Fatal error: Cannot redeclare get_plugin_dir() (previously declared in /home/imb/public_html/wp-content/plugins/wp-smush-pro/wp-smush.php:128) in /home/imb/public_html/wp-content/plugins/wp-smushit/wp-smush.php on line 120. How will I be able to resolve this issue