How to limit purchases to one per order

functions.php

<?php
add_filter( 'woocommerce_add_to_cart_validation', 'wc_limit_one_per_order', 10, 2 );
function wc_limit_one_per_order( $passed_validation, $product_id ) {
	if ( 31 !== $product_id ) {
		return $passed_validation;
	}

	if ( WC()->cart->get_cart_contents_count() >= 1 ) {
		wc_add_notice( __( 'This product cannot be purchased with other products. Please, empty your cart first and then add it again.', 'woocommerce' ), 'error' );
		return false;
	}

	return $passed_validation;
}