Kindly advise what the halacha is if one (mistakenly?) said kadesh for a grandparent who passed away on adar aleph, during this adar aleph, does one have to say kadish in adar beit again? Thank you.
If a parent passed away in Adar Aleph of a leap year, the appropriate time for saying kaddish is Adar Aleph.
If he passed away on Adar of a regular year, then according to the Shulchan Aruch (Orach Chaim 568:7) kaddish is recited on the second Adar of a leap year. However, the Rema adds that “some state that he should fast in the first [Adar], unless the demise was in the second Adar of a leap year, in which case one fasts in Adar Sheini. The custom is to fast in the first Adar. However, some are stringent and fast in both months” (see also Yoreh De’ah 402:2).
The Mishnah Berurah (42) confirms that it is proper to fast and to say kaddish in both months.
Therefore, for somebody Ashkenazi it is fine and proper to say kaddish in the first Adar month, but you should also say kaddish in the second Adar.