Easy Digital Downloads - Add specific download to cart if it doesn't already exist

gistfile1.php

<?php
/**
 * Add a specific download to the cart if not already there
 */
function sumobi_edd_add_download_to_cart_if_empty() {

	$download_id = 123;

	$cart = function_exists( 'edd_get_cart_contents' ) ? edd_get_cart_contents() : false;
 	
	if ( function_exists( 'edd_is_checkout' ) && edd_is_checkout() && ! $cart ) {
		edd_add_to_cart( $download_id, array( 'price_id' => 0 ) );
	}
}
add_action( 'template_redirect', 'sumobi_edd_add_download_to_cart_if_empty' );