I am adding a filter for woocommerce_checkout_fields but instead of updating the existing field label it creates a duplicate field in the woocommerce-additional-fields section as shown in the image.

$fields['order']['billing_phone']['label'] = 'Mobile number (in resort contact number)';

return $fields;

I understood that this should override the existing field label – https://docs.woocommerce.com/document/tutorial-customising-checkout-fields-using-actions-and-filters/#

Any pointers appreciated.