Plugin could not be activated because it triggered a fatal error.

I get this when trying to activate the plugin

Fatal error: Cannot redeclare wordpress_importer_init() (previously declared in /homepages/2/d647816794/htdocs/app647844131/wp-content/mu-plugins/wordpress-importer/wordpress-importer.php:1123) in /homepages/2/d647816794/htdocs/app647844131/wp-content/plugins/wordpress-importer/wordpress-importer.php on line 1209