Praise be to Allah.
If the husband is unsure as to whether he uttered the word of divorce at all, or as to the number of divorces he has uttered, then he should act on the basis of what is certain. So if he is not sure whether he has divorced her or not, the basic principle is that divorce has not taken place, because [in this instance] nikaah (marriage) is something which is certain, and divorce is something concerning which there is uncertainty. One of the basic principles is that certainty cannot be overridden by doubt. If he is uncertain as to whether he has divorced her once or twice, he should assume that he has divorced her once, because this is what is certain.