Tosfos (R"H 16b) asks why blowing shofar two times on Rosh HaShana -- tekiyos d'meyushav before musaf and then tekiyos again during the brachos of the amidah -- does not violate bal tosif. The Torah said to blow one set of tekiyos, not two! Tosfos answers that performing a mitzvah multiple times is not a violation of bal tosif, as we see from the case of a kohein who can duchen in one minyan and then duchen again should he go to another shul.
The Rashba gives what seems to be a simpler answer to Tosfos' question Rashba writes that the reason there is no bal tosif is because the Rabbis legislated that this is how we are to perform the mitzvah. Blowing shofar two times is no different than taking lulav on chol hamoed sukkos or fulfilling any other Rabbinic law which is categorically excluded from bal tosif.
Interestingly, these two views have elicited opposite reactions in various Achronim. The Pnei Yehoshua sees the Rashba's answer as so obvious he questions why Tosfos avoided it. The Turei Even, on the other hand, argues that the Rashba must agree with Tosfos. Take the case of eating matzah on Pesach -- surely no one eats exactly a k'zayis and no more. If only a Rabbinically legislated increase in shiur is permissible, how do our actions pass muster? (See this post from a few years ago for R' Chaim's explanation of the machlokes, also see here as well.)
R' Shach in Avi Ezri (Hil. Mamrim) suggests that Tosfos and Rashba are in agreement. Both answers are necessary because they are addressing different questions. As we learned last post, bal tosif can apply both to the chiyuv hamitzvah, the nature of the obligation, and the kiyum hamitzvah, its fulfillment. The Rashba's question focusses on the chiyuv of blowing shofar -- how could Chazal legislate sixty (or ninety or 100) kolos when the Torah mandated far fewer? The answer is that Rabbinic legislation is by definition not a modification of Torah chiyuv; it is just an additional obligation. The Turei Even's question from the case of eating more than a k'zayis of matzah is of no concern to the Rashba because Chazal never legislated how much to eat.
Tosfos' question relates to the kiyum mitzvah of shofar. Chazal changed the parameters of the obligation of shofar, but how do we justify in practice blowing more than the number of kolos mandated by the Torah? The answer is that fulfilling a mitzvah multiple times is not a violation of bal tosif.
While the lomdus sounds nice, the question that bothers me is given that Chazal have a right to add to the chiyuv of shofar, why would our obligation to fulfill the words of Chazal not be justification enough to explain why there is no bal tosif in our kiyum of 100 kolos? Why would Tosfos need to devise a new answer instead of simplying extending the answer of the Rashba to the realm of kiyum hamitzvah?