Is it permissible to propose to another girl if a Muslim man is already engaged to his brother's fiancée?

It is forbidden to propose over the proposal of a Muslim until the first suitor abandons his proposal or grants permission to the other. This is based on the saying of the Prophet, peace and blessings be upon him: "A man must not propose over the proposal of his brother until the suitor before him abandons it or the suitor gives him permission." The prohibition includes those who propose to one woman or more, and it is restricted to cases where it occurs after approval of the proposal. However, before approval, there is no prohibition. Consensus has been established on its prohibition after the competent woman or the guardian of a minor girl has given her consent.