Learn Dutch: Deze transactie duurt al een maand en is niet onbelangrijk. - This transaction has been ongoing for a month and is not unimportant.

This is a rhetorical device called litotes, which is common in professional or formal Dutch. By negating the opposite (onbelangrijk), you create a strong, deliberate understatement. It often carries more weight and sounds more thoughtful than simply saying it is heel belangrijk.

Why does the Dutch sentence use the present tense duurt while the English translation says "has been ongoing"?

In Dutch, if an action started in the past and is still happening right now, you use the present tense. Because the transaction is still ongoing, Dutch uses duurt. English requires the present perfect continuous ("has been ongoing"), which often tricks English speakers into incorrectly trying to use the Dutch perfect tense (heeft geduurd).

How exactly does al work in the phrase duurt al een maand?

While al literally means "already," when it is combined with the present tense to describe an ongoing action, it translates best as "for" or "for... now." In this sentence, it emphasizes the duration: "has been ongoing for a month now."

Why is there no subject after en? Shouldn't it be "en die is niet onbelangrijk"?

When two clauses are joined by en (and) and they share the exact same subject (Deze transactie), Dutch usually drops the subject in the second half to avoid repetition. Adding a pronoun isn't strictly wrong, but leaving it out is much more natural.