Fix for Product Add-Ons bypassing the Sold individually setting in WooCommerce.

product-addons-sold-individually.php

/**
 * Fix for Product Add-ons bypassing Sold individually setting
 * Add this to the theme's functions.php or as a custom plugin
 */
add_filter( 'woocommerce_add_to_cart_validation', 'enforce_sold_individually_with_addons', 999, 5 );

function enforce_sold_individually_with_addons( $passed, $product_id, $quantity, $variation_id = 0, $variations = array() ) {
    // Get the product
    $product = wc_get_product( $variation_id ? $variation_id : $product_id );
    
    // Only check if product is sold individually
    if ( ! $product || ! $product->is_sold_individually() ) {
        return $passed;
    }
    
    // Check if this product is already in cart (ignoring add-on variations)
    foreach ( WC()->cart->get_cart() as $cart_item ) {
        if ( $cart_item['product_id'] === $product_id ) {
            // If it's a variable product, also check variation
            if ( $variation_id && isset( $cart_item['variation_id'] ) && $cart_item['variation_id'] !== $variation_id ) {
                continue;
            }
            
            wc_add_notice( 
                sprintf( 
                    __( 'You cannot add another "%s" to your cart. This product is limited to one per order regardless of the options selected.' ), 
                    $product->get_name() 
                ), 
                'error' 
            );
            return false;
        }
    }
    
    return $passed;
}