2. Question: According to Rav Moshe Feinstein why can a man who already heard megilla read it for a woman who has not heard megilla yet even according to the opinion that she is not part of arvus, every Jew is responsible for another which is why one can fulfill a mitzvah for someone else even if he already fulfilled the mitzvah himself?
A. The Digul merivivah says that women didn’t accept responsibility for each other to make sure we accomplish mitzvos, at Har Gerizim and Har Eival when we received the blessings and curses before entering the land. According to this opinion a man cannot fulfill a mitzvah for a woman, like kiddush or megilla, if he already fulfilled the mitzvah himself. He can’t do the mitzvah again.
B. Reading the megilla publicises the miracle. There is a mitzvah you get when publicizing the miracle.
Answer: In the Igros Moshe (Orach Chaim first part chapter 190) Rav Moshe says that by megilla reading one who already read can help others fulfill the mitzvah even without the concept of arvus since the reading is to publicize the miracle and one who already fulfilled the mitzvah of reading still has a mitzvah to publicize the miracle as much as possible, so he still considered part of the mitzvah and can help others fulfill the mitzvah of reading megilla even if the concept of arvus doesn’t apply to them (See Dirshu Mishna Berura in back page 127, footnote 10).