function pw_edd_prevent_duplicate_cart_items( $download_id, $options ) {

	if( edd_item_in_cart( $download_id, $options ) ) {

		if( edd_is_ajax_enabled() && defined('DOING_AJAX') && DOING_AJAX ) {

			die('1');

		} else {

			wp_redirect( edd_get_checkout_uri() ); exit;

		}

	}

}

add_action( 'edd_pre_add_to_cart', 'pw_edd_prevent_duplicate_cart_items', 10, 2 );