Leap Years - Yechaveh Da'at 1:83

In this teshuva, Rav Ovadia Yosef discusses the issue of Jewish leap years, specifically which of the two Adar is the primary month and which is considered the additional one. He deals with two issues. First, when it comes to a bar mitzvah boy, the consensus among poskim seems to be that he should celebrate in the second Adar, as that is the true Adar and Adar Rishon is the add-on. However, he notes that if both the year of birth and the year of the bar mitzvah are leap years, then the boy should celebrate in whichever Adar he was born.

The second issue concerns the practice of observed the 7th of Adar (the yahrtzeit of Moshe Rabbeinu) as a day of learning/fasting. There is some debate over which Adar should be utilized for this observance, based on a machloket in the Yalkut Shimoni of when Moshe died. Ultimately, Rav Yosef favors observing it in the second Adar, based on the Gemara in Kiddushin 38a which teaches that we know that Moshe died on the 7th of Adar by counting backwards from the entry of the Jews into the Land of Canaan (the mourned for him for 30 days, plus the days it took to enter the land and prepare for Korban Pesach - see the relevant psukim in Yehoshua). As such, even if it was a leap year when Moshe died (which could be an anachronism), he definitely died in the month before Nissan, which would be the second Adar.

Rav Yosef parenthetically notes that, in a similar vein, one who is observing yahrtzeit for a parent who died in Adar also does so in Adar Sheni. However, if the parent died in a leap year then, similar to the case of bar mitzvah, the yahrtzeit follows whichever Adar the death occurred in.