We have mentioned in the previous Halacha that if one was in the midst of reciting the Amida prayer and then suddenly remembers that he has already prayed this prayer, one must stop praying immediately, for all of one’s blessings are considered blessings in vain. One may not intend for the remainder of one’s prayer to be a voluntary/donated prayer, for a voluntary prayer is only applicable when one recites the entire prayer on a voluntary basis; however, one cannot pray half a prayer on an obligatory basis and the other half on a voluntary basis.
Indeed, the Rambam (Chapter 10 of Hilchot Tefillah) likewise rules in accordance with the majority of the Poskim that one who begins praying and then remembers that he has already recited this prayer must immediately stop praying and may not conclude it on a voluntary basis.
Nevertheless, the Rambam writes that if this happens during the Arvit prayer, i.e. if one began praying the Arvit Amida and then suddenly remembers that he has already prayed Arvit, one may, in fact, continue one’s prayer on a voluntary basis. The reason for this is because there is a distinction between the Shacharit and Mincha prayers versus the Arvit prayer, for the Gemara states that Arvit was only established by our Sages as a voluntary prayer and not as an obligatory prayer. Thus, whenever one prays Arvit, this is not being done on a fully obligatory basis and one is merely praying on a voluntary basis. Thus, if one remembers in the middle of reciting the Arvit prayer that one has already prayed, one may continue this prayer, for even beforehand, this prayer was likewise only on a voluntary basis.
On the other hand, Rabbeinu Mano’ach rebuffs the Rambam’s opinion, for although our Sages originally established the Arvit prayer as a voluntary prayer, later, this prayer was established in an obligatory manner and every Jewish man must pray Arvit every day. If so, nowadays, there is no halachic distinction between the Shacharit, Mincha, and Arvit prayers. Some write that the Ra’avad agrees to Rabbeinu Mano’ach’s approach in that the entire Jewish nation has accepted the Arvit prayer as an obligatory prayer and there is no reason to differentiate between one prayer and another.
Nevertheless, Maran Ha’Shulchan Aruch writes in his commentary on the Rambam, Kesef Mishneh, that this reasoning is insufficient to rebuff the words of the Rambam, for originally Arvit was established as a voluntary prayer and thus ultimately, it does not share the same status as the other prayers which were established as completely obligatory prayers already in the times of our Sages and there is thus room to uphold the opinion of the Rambam in this regard. Therefore, it seems that the ruling on this issue follows the Rambam and if one begins praying Arvit and then remembers that he has already recited this prayer, one may continue one’s prayer on a voluntary basis.
However, in his Shulchan Aruch, Maran makes no mention of a distinction between Shacharit and Mincha and Arvit. It would therefore seem that Maran means to rule in accordance with the opinion of Rabbeinu Mano’ach and the Ra’avad, unlike the Rambam. The Magen Avraham, Rabbeinu Yehuda Ayash, and others interpret the words of the Shulchan Aruch in this manner.
Nonetheless, Maran Rabbeinu Ovadia Yosef zt”l proves beyond a shadow of a doubt that the Ra’avad does not disagree with the Rambam regarding this issue and he writes that even according to the Ra’avad, one may continue one’s Arvit prayer on a voluntary basis. He proceeds to quote several other Poskim who rule likewise and write that even according to Maran Ha’Shulchan Aruch, the Halacha follows the opinion of the Rambam and if one begins reciting the Arvit Amida prayer and then remembers that he has already recited this prayer, one may continue with the Amida prayer on a voluntary basis.
Summary: One who begins the Arvit prayer and then remembers that he has already prayed Arvit may continue reciting the Amida on a voluntary basis.