How can one reconcile the prohibition against a man proposing to a woman already proposed to by his brother with the story of the woman consulting the Prophet (peace be upon him) regarding Mu'awiya and Abu al-Jahm's simultaneous proposals to her?

The prohibition against proposing to a woman who has already received a proposal is understood to apply after the woman has accepted and agreed. However, before her acceptance, there is no harm in proposing to her. This is evidenced by Fatimah bint Qais's consultation with the Prophet, peace and blessings be upon him, regarding her suitors before she had agreed to any of them. There is a consensus (ijma') on the prohibition of proposing to a woman who has already received a proposal after the explicit acceptance of the legally competent woman or the guardian of a minor girl, provided the suitor is suitable.