How add a group by to the query used by the media library?

What would be the proper way to add a group by to the media library query? What I have tired so far is not working:

add_filter('posts_groupby ', 'limitMediaLibraryItems', 10, 2 );
function limitMediaLibraryItems($groupby) {

    if ( strpos( $_SERVER[ 'REQUEST_URI' ], '/wp-admin/upload.php' ) !== false ) {    
        $groupby .= " guid";
    }

    return $groupby;
}