If a congregation mistakenly read on one day of Hanukah the weekly Parashat Ha’shabua, instead of the reading for Hanukah, and they read three Aliyot, they have fulfilled their obligation and do not have to then read the reading for Hanukah. When it comes to Hanukah, Halacha does not require a congregation to read the special Hanukah reading if it had mistakenly read the regular Torah portion.
When Rosh Hodesh Tebet (which always occurs during Hanukah) falls on a weekday, two Sifreh Torah are removed from the ark. The congregation reads three Aliyot for the Rosh Hodesh reading from the first scroll, and then calls a fourth person for the Hanukah reading from the second Sefer. This follows the famous Halachic principle of "Tadir Ve’she’eno Tadir, Tadir Kodem," which means that when we have two Misvot to perform, we first perform the more frequent of the two Misvot. Since the Rosh Hodesh reading is more frequent than the Hanukah reading, we first read the Rosh Hodesh reading and then the Hanukah reading.
If the reader mistakenly read the Hanukah reading from the first Sefer Torah, then he reads three Aliyot for the Rosh Hodesh reading from the second scroll. If the reader mistakenly read four Aliyot for the Rosh Hodesh reading from the first Sefer Torah, which is the number of Aliyot customarily read for the Rosh Hodesh reading, the congregation should call a fifth person and make another Aliya for the Hanukah reading from the second scroll. Once a Sefer Torah is taken from the ark, it is disrespectful to return it to the ark without reading from it, which gives the impression that it is invalid for use. Therefore, an additional Aliya must be called in order to read from the second Sefer Torah. This is the ruling of the Shulhan Aruch (Orah Haim 684:3).
(Based on Yalkut Yosef, p. 207)
