Suppose on a Saturday night, someone says shmoneh esrei with "ata chonantanu" but forgets another part that requires repeating shmoneh esrei (like ya'aleh v'yavo on chol hamoed). When he repeats shmoneh esrei, should he say "ata chonantanu" again?
As quoted here according to Rav Tzvi Frumer (in Eretz Tvi 24), this depends on which side of a Machlokes Rishonim we pasken on the question of if someone forgot Yaleh VeYavo on Rosh Chodesh Mincha, if he has to daven the Amida twice during Ma'ariv.
One view (Rabbeinu Yona) is that missing Yaaleh VeYavo does not mean one didn't pray, there was just something missing. So there is no need to say the Amida twice. According to that view, the Attah Chonantanu was said as part of a proper prayer, and thus would not need to be repeated.
The second view (Brought in Tosfos, with support from the Rif) is that missing Yaaleh VeYavo invalidates the whole Amida and it is as if he never said it, so he has to say the Amida twice. According to this, the Attah Chonantanu was not properly said and would have to be repeated.
The Rosh paskens (with regards to Motzei Rosh Chodesh) to repeat it out of concern for the other opinion as a voluntary prayer, and this is the Halacha in Shulchan Aruch (O.C. 108).
Note, however, that the Magain Avraham argues that if the Mincha where Yaleh VeYavo is forgotten is the first day of Rosh Chodesh, then both opinions hold that you have to repeat the Amida in Ma'ariv as you can then say Yale VeYavo. Shulchan Aruch HaRav and the Mishna Brura both pasken this way. So it would seem, לענ"ד that according to that, since you can say Ata Chonantanu in Ma'ariv in this case, you have to, even according to the other opinion. You could argue and distinguish though - because Ata Chonantanu is not something that Shemona Esrei is repeated over, unlike the case of Mincha Rosh Chodesh, so perhaps Rabbeinu Yona would hold that you don't have to repeat it because of that, even according to the Magain Avraham.