Prevent Duplicate Items in Cart

<?php
if ( ! class_exists( 'WooCommerce' ) ) {
    return;
}

add_filter( 'woocommerce_add_to_cart_validation', 'scseed_prevent_duplicate_cart_items', 10, 2 );
function scseed_prevent_duplicate_cart_items( $passed, $product_id ) {
    foreach ( WC()->cart->get_cart() as $cart_item ) {
        if ( (int) $cart_item['product_id'] === (int) $product_id ) {
            wc_add_notice(
                sprintf(
                    /* translators: %s: link to the cart page */
                    __( 'This item is already in your cart. <a href="%s">View cart</a>', 'woocommerce' ),
                    esc_url( wc_get_cart_url() )
                ),
                'error'
            );
            return false;
        }
    }
    return $passed;
}