What is the ruling on proposing to a woman who has already been proposed to by another man?

It is forbidden to propose to a woman who has accepted another man's proposal, unless the first man withdraws or gives permission.
The Prophet ﷺ said that a man may not propose over his brother's proposal until the earlier suitor abandons it or permits him. This prohibition protects feelings, dignity and good relations between families. In the Shafi'i school, the prohibition applies once the first proposal has been clearly accepted by the woman or her wali. If the first proposal was declined, or no answer has yet been given, another man may propose. The evidence is the hadith of Fatimah bint Qais, who was proposed to by two men at once, and the Prophet ﷺ advised her to choose Usamah. If someone nonetheless proposes and marries in that forbidden situation, he is sinful but the marriage is valid according to the majority.