It seems early to start thinking of Rosh haShana, but we are getting closer. The Sha'agas Arye (103) asks whether it is permissible to blow shofar for no reason after one has already fulfilled the obligation of tekiya on Rosh haShana. On a regular Yom Tov there is an issur derabbanan of blowing shofar, just like there is an issur derabbanan of playing a flute. At first glance one might argue that one has already fulfilled the mitzvah of shofar, this issur derabbanan prohibits further use of the shofar.
The Sha'agas Arye writes that if one has a safeik whether one fulfilled the mitzvah of shofar or not, it is permissible to blow shofar again not withstanding the potential violation of an issur derabbanan. This seems like an obvious point, but the Sha'agas Arye brings a ra'aya that is not so obvious. He compares this case to R' Abahu's takanah to blow TSHR"T, TSH"T, and TR"T to be yotzei every possible safeik of what a shevarim is.
Two points on his analogy: 1) R' Abahu made a takanah institutionalizing the heter to blow even in a case of safeik. There obviously is no issur derabbanan if the rabbanan told you to blow! How is this comparable to a case where an individual has a safeik whether he fulfilled the mitzvah, in which case there is no special takanah? 2) The Ran (end of Rosh haShana) writes that there was never a real safeik whether shevarim was shevarim, teru'ah, or a combination. There were multiple equally valid minhagim how to blow and R' Abahu simply standardized the practice to blow all the valid combinations. Since there was never a real safeik to begin with (though other Rishonim may disagree), how is the comparison valid?