oomis: you are quite right. The minhag-certainly amongst the chassidic crowd- is to observe an adar yahrzeit on both adars-when there is a leap year. Obviously, if there is only one adar, then the yahrzeit -regardless whether the year of death was a leap year or not-is observed on that adar. I am not sure what happens when the death occurred on a leap year, when does the yahrzeit falls on a leap year? Do you only observe the adar when the death occured-or do you also then observe both adars?
I will research that! BTW- a similar situation exists with bar mitzvahs in adar- although,of course, there is only one bar mitzvah celebration. It can be that a boy born after another boy (say on adar sheni) has a bar mitzvah before another who was born earlier in a leap year!