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Why do we read the Megillah by night and then again by day? Wouldn't it be enough to read it once!

The Talmud in Megillah 4a lays out the requirement for reading the Megillah both at night and during the day:

ואריב"ל חייב אדם לקרות את המגילה בלילה ולשנותה ביום שנאמר אלהי אקרא יומם ולא תענה ולילה ולא דומיה לי סבור מינה למקרייה בליליא ולמיתנא מתניתין דידה ביממא אמר להו רבי ירמיה לדידי מיפרשא לי מיניה דרבי חייא בר אבא כגון דאמרי אינשי אעבור פרשתא דא ואתנייה איתמר נמי אמר רבי חלבו אמר עולא ביראה חייב אדם לקרות את המגילה בלילה ולשנותה ביום שנאמר למען יזמרך כבוד ולא ידום ה' אלהי לעולם אודך

R. Joshua b. Levi further said: It is the duty of a man to read the Megillah in the evening and to
repeat it in the day, as it is written, O my God, I cry in the daytime, but thou answerest not, and in
the night season and am not silent. The students took this to mean that the [Megillah] should be
read at night, and the Mishnah relating to it should be learnt in the morning. R. Jeremiah. however,
said to them: It has been explained to me by R. Hiyya b. Abba [that the word ‘repeat’ here has the
same meaning] as when, for instance, men say, I will go through this section and repeat it. It has also
been stated: R. Helbo said in the name of ‘Ulla of Biri: It is a man's duty to recite the Megillah at
night and to repeat it the next day, as it says, To the end that my glory may sing praise to thee [by
day]. and not be silent [by night]. O Lord, my God, I will give thanks to thee for ever. (Soncino translation)

Rashi, commenting on the words "repeat it in the day", explains:

זכר לנס שהיו זועקין בימי צרתן יום ולילה

A remembrance of the miracle, for they cried out in the time of their distress day and night.

Apparently, then, reading the Megillah at night and during the day is meant to correspond to the crying out that occurred both at night and during the day.

This reason is cited by R. Joseph Karo in his commentary to R. Jacob Ben Asher's codification of this law in Tur O.C. 687.