'Uqbah bin 'Amir (May Allah be pleased with him) said:
The Messenger of Allah (ﷺ) said, "A Mu'min is the brother of another Mu'min; and thus it is not permissible for a Mu'min to make an offer while the offer of his brother is pending, nor should he make a proposal of marriage while that of his brother is pending till he withdraws his proposal."[Muslim].
App reference: Book 18, Hadith 270
Arabic/English book reference: Book 18, Hadith 1780