Empty cart before add to cart in WooCommerce

Updated (with 2 different alternatives):

You should try the following:

add_filter( 'woocommerce_add_to_cart_validation', 'remove_cart_item_before_add_to_cart', 20, 3 );
function remove_cart_item_before_add_to_cart( $passed, $product_id, $quantity ) {
    if( ! WC()->cart->is_empty() )
        WC()->cart->empty_cart();
    return $passed;
}

Code goes in functions.php file of your active child theme (or theme). Tested and works with both ajax-add-to-cart and normal add-to-cart…

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Or you can use this different one that will keep in cart the last added item:

// Keep only last cart item
add_action( 'woocommerce_before_calculate_totals', 'keep_only_last_cart_item', 30, 1 );
function keep_only_last_cart_item( $cart ) {
    if ( is_admin() && ! defined( 'DOING_AJAX' ) )
        return;

    if ( did_action( 'woocommerce_before_calculate_totals' ) >= 2 )
        return;

    $cart_items = $cart->get_cart();

    if( count( $cart_items ) > 1 ){
        $cart_item_keys = array_keys( $cart_items );
        $cart->remove_cart_item( reset($cart_item_keys) );
    }
}

Code goes in function.php file of your active child theme (or theme). Tested and works