1) One who erred in his prayer and did not say “Vesein Tal Umatar Livrachah”, and by the time that he realizes his error the time for praying has already passed, must pray the following Amidah twice. An example of this is when he forgot to say it in the afternoon prayers and the day has passed when he is obligated to pray the Amidah of the evening service twice over; the first one as usual and the second one as a replacement for the afternoon service.
2) However, if he forgot to say “Vesein Tal Umatar Livrachah”, in the afternoon service on Friday and Shabbos has already commenced, he should not pray the evening service of Shabbos twice, as on Shabbos there is no “Vesein Tal Umatar Livrachah” at all. He can also not pray a voluntary “generous” prayer as this is not permitted on Shabbos. Instead, he should listen to the repetition of the cantor of the blessing which is a shorter form of the seven blessings of the Shabbos Amidah, and request from the cantor that he should intend to allow him to fulfil his obligation by listening, and he himself should also concentrate on fulfilling his obligation when listening. (Shulchan Aruch and Mishne Berurah 108).