All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, is His slave and Messenger.
According to the adopted opinion of the Hanbali school, if someone marries a woman during her ‘Iddah (waiting period), she does not become permanently prohibited for him in marriage. Rather, it is permissible for him to marry her after she observes the ‘Iddah period from her first marriage and another ‘Iddah from the second marriage (with him). The second marriage is void because it was conducted whilst she was still observing her first ‘Iddah, but she is still required to observe a ‘Iddah for it. This was stated by Ibn Qudaamah, in Al-Mughni when he listed the opinions of the scholars who allowed this and those who prohibited it.
We would like to point out that the woman being in her ‘Iddah is one of the impediments to marriage. Therefore, it is prohibited to marry a woman during her ‘Iddah. Ibn Qudaamah reported in Al-Mughni that there is a consensus among the scholars in this regard.
Allah Knows best.