[Product Add-ons] Allow one instance of Sold Individually products in the cart, even if they have different add-ons selected

wc-product-addons-sold-individually.php

<?php

add_filter( 'woocommerce_add_to_cart_validation', 'check_if_product_exists_in_cart', 10, 2 );

function check_if_product_exists_in_cart( $is_valid, $product_id ) {

	$product = wc_get_product( $product_id );

	if ( $product->is_sold_individually() ) {

		$cart_contents = WC()->cart->get_cart_contents();

		foreach ( $cart_contents as $cart_item ) {

			if ( $product_id === $cart_item[ 'product_id' ] ) {
				$is_valid = false;
				$notice   = sprintf( '<a href="%s" class="button wc-forward">%s</a> %s', wc_get_cart_url(), __( 'View cart', 'woocommerce' ), sprintf( __( 'You cannot add another "%s" to your cart.', 'woocommerce' ), $product->get_name() ) );
				wc_add_notice( $notice, 'error' );
				break;
			}
		}
	}

	return $is_valid;
}