Add only one product to cart

<?php 

add_filter( 'woocommerce_add_to_cart_redirect', function() {
   return wc_get_checkout_url();
});

add_filter( 'wc_add_to_cart_message_html' , function() {
   return '';
});
 

add_filter( 'woocommerce_add_to_cart_validation', function( $passed, $product_id, $quantity ) {
    if( ! WC()->cart->is_empty() ) {
        WC()->cart->empty_cart();
    }

    return $passed;
}
, 20, 3 );

remove_action( 'woocommerce_order_details_after_order_table', 'woocommerce_order_again_button' );