Does the marriage of one who proposes to the betrothed of his Muslim brother suffer negative consequences, and will they be punished for their transgression? What is your advice and warning to those who deliberately commit such an act, given the prevalence of this phenomenon?

It is forbidden to propose marriage when another Muslim has already proposed, based on the Prophet's (peace and blessings be upon him) saying: "A man should not propose marriage when his brother has already proposed until he [the first suitor] marries or abandons [the proposal]." Some scholars have mentioned that a woman's or her guardian's rejection of a suitor after a mutual inclination has been established is not forbidden, unless the rejection is for the sake of a second proposal. As for the worldly consequence of this sin, it is not necessarily immediate, but it is an injustice to the suitor, and the supplication of the oppressed is answered. One must beware of oppressing others, and remember that whoever abandons something for the sake of Allah, Allah will compensate him with something better. Everything that happens to a servant is predestined and written, but predestination should not be used as an excuse for committing sins. Rather, it should be used to alleviate sorrow over what has passed, and to be certain that what befell you could not have missed you, and that Allah knows what is good. Allah Almighty says: "But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah knows, while you know not."