If this occurred on a weekday, one must recite the given Tefila (Shulchan Aruch 107:1), and he should have in mind that if he had already prayed, this prayer should be deemed a voluntary prayer (Mishna Berura 107:2). On Shabbat or Yom Tov, one does not recite the Amida in such a case (Shulchan Aruch 107:1).
He should stop right where he is, and should not complete even the Beracha that he currently recites (Shulchan Aruch 107:1). If this occurred during Arvit, however, he may continue the Amida and have in mind that it should be considered a voluntary prayer; in such a case, he should add a new request in each of the middle Berachot of the Amida (Halacha Berura).
No, because nowadays we do not allow reciting voluntary Tefilot unless one is uncertain whether or not he had fulfilled his basic obligation to pray (Halacha Berura).
No; he stops immediately at the point where he realizes his mistake (Halacha Berura). If this occurred during Arvit, however, he may continue the Amida and have in mind that it should be considered a voluntary prayer; in such a case, he should add a new request in each of the middle Berachot of the Amida (Halacha Berura).
Yes, and he should not intend for it to be considered a voluntary prayer if he had already prayed; since he recites the Amida on behalf of the congregation, he should have in mind that he recites an obligatory Tefila (Halacha Berura).
If this occurred on a weekday, one must recite the given Tefila (Shulchan Aruch 107:1), and he should have in mind that if he had already prayed, this prayer should be deemed a voluntary prayer (Mishna Berura 107:2). On Shabbat or Yom Tov, one does not recite the Amida in such a case (Shulchan Aruch 107:1).
He should stop right where he is, and should not complete even the Beracha that he currently recites (Shulchan Aruch 107:1). If this occurred during Arvit, however, he may continue the Amida and have in mind that it should be considered a voluntary prayer; in such a case, he should add a new request in each of the middle Berachot of the Amida (Halacha Berura).
No, because nowadays we do not allow reciting voluntary Tefilot unless one is uncertain whether or not he had fulfilled his basic obligation to pray (Halacha Berura).
No; he stops immediately at the point where he realizes his mistake (Halacha Berura). If this occurred during Arvit, however, he may continue the Amida and have in mind that it should be considered a voluntary prayer; in such a case, he should add a new request in each of the middle Berachot of the Amida (Halacha Berura).
Yes, and he should not intend for it to be considered a voluntary prayer if he had already prayed; since he recites the Amida on behalf of the congregation, he should have in mind that he recites an obligatory Tefila (Halacha Berura).