Is it forbidden to marry an engaged woman, even with her knowledge and consent?

If a man proposes to a girl, and then another man proposes to her, unaware of the first proposal, he bears no sin due to his ignorance, for error is excused. If an agreement is reached between the second suitor and the girl, the first suitor is not permitted to propose to her again except with his permission or if the engagement is annulled. The girl has the right to annul the engagement whenever she wishes. One must be cautious of sabotaging an engagement. Some scholars have gone so far as to say that if someone proposes over another's proposal and marries her, their marriage is annulled if they have not yet consummated it. It is advisable to seek someone else.