IIRC, During a Judaic leap year, my father, a"h observed 2 Yahrtzeits for his father who died during Adar of a non-leap year. He lit the yahrtzeit candle and said Kaddish twice - once for the date in Adar 1 and again on the same date in Adar 2.
I'm not sure what the halacha or common minhag is.
The phrasing of Talmud Megilla 1:4 implies that Adar 1 and Adar 2 are identical except regarding Megillah reading and giving of alms to the poor.
To me, this implies that every yahrtzeit observed in Adar, regardless of whether the person died in a leap year or non-leap year, would be observed twice.
Is this what is done? Does the Mishnah imply that yahrtzeit observance should be included twice?