All other shabbatot have unique torah readings (aside from the regular cycle, of course), but Shabbat during Sukkot and Shabbat during Pesach read the same passage from Ki Tisa (with varying maftir only). Why is this -- why the duplication and, secondarily, why that particular passage?
I am not asking what one can learn from this; I'm asking why the rabbis made the decisions they did.
The source is in the Talmud, Megillah 31a (link). It is a statement of Rav Huna, in the name of Rav:
אמר רב הונא אמר רב שבת שחל להיות בחולו של מועד בין בפסח בין בסוכות מקרא קרינן ראה אתה
Rashi explains:
מקרא קרינן ראה אתה - שיש שם מצות שבת ורגלים וחולו של מועד דכתיב את חג המצות תשמור ומכאן למדנו איסור מלאכת חולו של מועד במסכת חגיגה (דף יח.):
My free translation: The portion we read is "See, You" (Shmot 33:12) -- Which has in it the mitzvot of Shabbat, and the pilgrimage festivals, and chol hamo`ed, as it is written (34:18): "Keep the festival of Matzot," and from this we learn the prohibition of work of chol hamo`ed, in Tractate Chagiga (18a).
So, it's not a question of "duplication," exactly; rather, this passage mentions Shabbat, the holidays, and the pasuk which is the derivation of chol hamo`ed's prohibition of work. This section of the Torah is simply the best fit for Shabbat chol hamo`ed, whether during Pesach or Sukkot.
[Now, you might say, well, according to that Gemara in Chagiga, there are three (or five) opinions for the source of prohibiting work on chol hamo`ed. Why not read one of the other sources? I don't have an answer for that.]
