What is the benefit of mentioning "prophet" after "messenger" in the verse: (And mention in the Book Ishmael. Indeed, he was truthful in [his] promise, and he was a messenger, a prophet), knowing that every messenger is a prophet?

The well-known distinction between a Prophet (Nabi) and a Messenger (Rasul) is that a Messenger is divinely inspired with a law and commanded to convey it, while a Prophet is divinely inspired with a law but not commanded to convey it.

The more evident view is that a Messenger is sent to a disbelieving people who deny the truth, whereas a Prophet is sent to a believing people who adhere to the law of a Messenger before him, teaching them and judging among them.

It has been established that Noah was the first Messenger sent to the people of the earth. Before him, there were Prophets such as Seth and Enoch, and Adam was a Prophet who communicated with Allah.

Allah Almighty's saying about Ishmael: ﴿And mention in the Book Ishmael. Indeed, he was truthful in [his] promise, and he was a messenger and a prophet.﴾ (Maryam/54) means that he was a Messenger to the Jurhum tribe, conveying the law of his father Abraham, and a Prophet informing them of that law. The combination of the two descriptions is an honor for him and an affirmation of his status.