The antonyms for the phrase "be one duty to" include phrases like "not obligated to", "not responsible for", "not obliged to", "not required to", and "not duty-bound to". These phrases indicate a lack of responsibility or obligation to complete a certain task or duty. It is important to note that while these phrases may be antonyms for "be one duty to", they do not necessarily mean that the task or duty should be ignored or neglected. Rather, they indicate that there is a lack of obligation or duty to complete the task, leaving it up to personal choice or discretion.