Is it considered an irrevocable or a second divorce, given that the wife knew she was menstruating during a previous divorce?

A woman is not permitted to seek a divorce unless there is a valid excuse, for the Messenger of Allah (peace be upon him) said: "Any woman who asks her husband for divorce without any strong reason, the fragrance of Paradise will be forbidden to her."

If a man divorces his wife twice, whether with a single utterance or two utterances, in one sitting or two sittings, only one divorce is considered to have occurred, according to the preponderant opinion. No divorce after a divorce can take place unless there is a reconciliation (revocation of divorce) or a new marriage contract after the 'iddah (waiting period).

Therefore, what is mentioned in the question constitutes two divorces, and one divorce remains.