Tosfos in Rosh haShana (16) asks why blowing shofar twice, once during musaf and once before musaf, as instituted by Chazal, does not violate bal tosif, the issur of adding to the Torah - there is only one mitzvah of shofar, not two? The simplest answer to Tosfos’ question is offered by the Rashba: by definition, Rabbinic enactments do not violate bal tosif; they are not new, additional mitzvos being added to the Torah, but are categorically included in the Torah granted right of Chazal to legislate. Tosfos does not give this obvious answer and instead suggests that the repitition of a mitzvah (without adding anything to it) is not a violation of bal tosif; e.g. a kohen can fulfill the mitzvah of birchas kohanin in one shule and then repeat the bracha again in another shule with no harm – only adding to the Torah’s formula of the bracha would constitute bal tosif. Why did Tosfos avoid giving the Rashba’s answer?
One possibility is that tekiyos before musaf are described as a minhag. Perhaps Tosfos accepts the Rashba’s idea, but limits it to Rabbinic law, not minhag. Mike S. in a comment in a different context suggested that there may be a distinction between gezeiros and takanos. Perhaps this distinction applies here: Tosfos may accept that the Rabbis have legislative authority to make a gezeirah to protect Torah law, but see as more problematic the institution of a completely new takanah (see PM”G, Pesicha haKolleles 1:40 who suggests such a distinction). The Pnei Yehoshua similarly distinguishes two types of Rabbinic laws – laws which historical necessity forced the Rabbis to implement to protect the Torah, and laws which could have been enacted at any time/place and are completely independent of historical circumstance. In the latter case, since these laws could theoretically have existed from the moment the Torah was given, their absence, their not having been given or formulated earlier, is itself indicative of that fact that these enactments are not needed and would be an addition violating bal tosif.