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One who did not hear the Megillah on the 14th: 
One who did not hear Megillah on the 14th, such as he was traveling and did not have a Megillah available, then if a Megillah becomes available on the 15th, he is to read it on the 15th [one time at night and one time by day. The readings are to be done without a blessing. On the 16th and onwards one is not to read the Megillah at all, even without a blessing.]
What is the law if one accidently read the Megillah in the first Adar of a leap year?
One must repeat the reading in Adar II.
 Michaber 688/8
 P”M 688 A”A 12; Kaf Hachaim 688/59
 M”A 688/12; Peri Chadash; Mateh Yehuda; Nehar Shalom 688/6; Peri Megadim 688 A”A 12; M”B 688/23; Shulchan Gavoa 688/25; Kaf Hachaim 688/5.
The reason: As the main time of the reading has passed. [M”B ibid]
Other Opinions: Some Poskim rule that one is to read the Megillah with a blessing. [Taz 688/9; Elya Raba 688]
 M”B 688/23
The reason: The verse [Esther 9/27] states “Velo Yaavor”. This teaches us that the Megillah may not be read past the 15th. [See Rashi Megillah 2a “Chal Arba Asar” regarding the reason why the Megillah is read on Friday by Purim Mesulash; Likkutei Sichos 17 p 69; Shulchan Menachem 3/331] Obviously however this only refers to one reading from a Kosher Megillah for the sake of the Mitzvah.
 Darkei Moshe 688/7; M”A 688/11; M”B 688/23; Kaf Hachaim 688/60