The prohibition of proposing marriage when one's brother has already proposed, unless he gives permission or gives up the idea Sahih Muslim 1412 a

Sahih Muslim 1412a
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Translation
Ibn Umar (Allah be pleased with them) reported Allah's Apostle (ﷺ) as having said this

None amongst you should outbid another in a transaction, nor should he make proposals of marriage upon the proposal made by someone else.