how to exclude posts on current page from recent posts sidebar widget

Add this to the functions.php theme (or child theme) file.

add_filter( 'widget_posts_args', 'wpse_109484_recent_post_count' );

function wpse_109484_recent_post_count( $args ) {
    global $wp_query;

    $excluded_posts = array();
    foreach ( (array) $wp_query->posts as $post ) {
        $excluded_posts[] = $post->ID;
    }

    $args['post__not_in'] = $excluded_posts;

    return $args;
}