Must the second husband ensure that his wife has completed her legal waiting period (iddah) after her divorce from her first husband, and what are the consequences if she has not completed it?

The default status for sexual relations is prohibition. A married woman remains under the legal authority of her husband until her divorce or khul' (divorce at the wife's instance, usually with compensation) is proven with certainty. This cannot be established by her word alone, but rather by two witnesses or the husband's admission. The husband, therefore, was obligated to ensure her lawful separation and the completion of her iddah (waiting period), which is three menstrual cycles (except for a woman who has despaired of menstruation, for whom it is three months). If it becomes clear that her divorce or khul' was not valid, he must separate from her because she is still under the legal authority of her first husband, and his marriage to her is فاسد (invalid). She would then have two iddahs that do not overlap.