LB, for those living in Yerushalayim (or other walled cities) it is not an extra reading, but their real ones!
The real interesting Purim is for those living in places where it is a safek if it is walled or not, or if it is part of the walled city or not. There are different opinions for those places- including keeping 2 days of Purim, meaning 4 megilla readings, 2 seudos, mishloach manos and matanos l’evyonim 2 days in a row.
And then there is Purim Meshulash- unique for the walled cities, when Shushan Purim comes out on shabbos, then it is split into 3 days with megilla and maataos l’evyonim on Purim/Friday; Seuda, misloach manos on Sunday and on shabbos, shushan Purim, daven like on Purim – with al hanisim and special Purim leining.