Redirect posts to canonical URL when accessed via secondary category URL slugs

redirect_to_canonical.php

<?php
/*
Plugin Name: Redirect to Canonical
Description: Redirect posts to canonical URL when accessed via secondary category URL slugs.
Version: 1.0
Author: Greg Perham
License: GPL2
*/

defined( 'ABSPATH' ) || exit;


/* https://wordpress.stackexchange.com/questions/19290/how-can-i-force-wordpress-to-redirect-to-canonical-permalinks#answer-410192 */
add_action( 'template_redirect', 'r2c_canonical_redirect' );
function r2c_canonical_redirect() {
	if ( is_singular( 'post' ) ) {
		global $wp;
		$wp->parse_request();
		 $current_url = trim( home_url( $wp->request ), '/' );
		 $canonical = wp_get_canonical_url();
		 $surl = trim( $canonical, '/' );
		if ( $current_url != $surl ) {
			wp_redirect( $canonical, 301 );
			exit;
		}
	}
}