Shulchan Arukh, Orach Chayim 282/שולחן ערוך·אורח חיים רפ״ב

1We bring out a Torah scroll and seven read from it. And if one wants to add [to the number of readings], one adds. Gloss: And this is the law: on a Festival, it is permitted to add to the count of readers (Rambam, The Laws of Prayer and Repentence, chapter 12; Maharam; and Beit Yosef). And there are those who say that on a Festival one may not add (Ran, chapter "Ha-Korei"). And thus (i.e. the latter) they practiced in these countries (i.e. Ashkenaz), except for on Simchat Torah when we add many. Also on every Festival, the Maftir is not [included in] the count of readers, as seen below.

2It is permitted to read [the same readings for] those who are called up to the Torah multiple times, even though this one read what this [other] one has [already] read and goes back and makes a blessing - there is nothing [problematic] with this. Gloss: And there are some who forbid [this] (Mordekhai, Sof Megillah) And thus they practiced in these countries (i.e. Ashkenaz) except for on Simchat Torah they practiced to multiply the readers and practiced according to the first opinion.