It is prohibited for a Muslim man to propose to a woman already betrothed by his Muslim brother, unless the latter marries. Does this ruling apply also if the first suitor betrothed another woman‎?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Proposing to a woman already betrothed by another man is prohibited, unless the first suitor breaks the betrothal or gives the permission to the other man. Ibn ‘Umar   reported that the Prophet, sallallaahu ‘alayhi wa sallam, said

…a man should not propose to a woman who is already engaged to his Muslim brother, unless the first suitor gives her up, or permits him to propose to her... [Al-Bukhari]

This ruling applies whether the first man is engaged to one woman or more.

However, the prohibition applies if this happened after accepting the betrothal of the first suitor, not before that.

Ibn Al-Ameer Al-San'aani said:

This is consensually prohibited only after accepting the proposal of the suitable suitor by an adult woman, or the Wali (marriage ‎guardian) in case of a young girl. As for the unsuitable suitor, the Wali must agree, according the opinion that gives him the right to refuse. Approval must be explicit, because if approval is implicit, it will not be prohibited for another man to propose to that same woman or girl, according to the preponderant opinion.

Allaah Knows best.