The prophet SAW said: “no man should offer a proposal of marriage over the proposal of his brother until the first one marries her or gives him permission.” (Reported by al-Bukhaari)
In another version of the narration, The prophet saw said: “None of you should ask for a woman’s hand in marriage when his brother has already done so until the one who has already proposed leaves off his desire to marry her, or he gives him permission.” (Reported by Imam Ahmad and An-nasaai)
If a woman is ascribed to a suitor, it is unlawful for another man (who is aware of the presence of a suitor) to propose to her unless the first proposal is canceled. It can lead to seriously hurting the first suitor and it can bring about enmity among people. This was a reason the Prophet SAW forbade a man from proposing where his brother had proposed. Ibn Qudaamah said: “regarding this issue, we do not know of any controversy among scholars”