The Shulhan 'Arukh states that the Berakha (blessing) of Sheheheyanu is said at night, over the reading of the Meghillah, but when it is read again during the day it is not repeated. This is the custom followed by Sephardim. The Rama adds that the custom in Ashkenazic lands is to repeat the Berakha of Sheheheyanu when reading during the day.
When one hears the Berakha of Sheheheyanu being recited over the reading of the Meghillah, it is appropriate to have in mind that it covers the Se'uddah (festive meal) and the sending of Mishlowah Manoth as well. Sephardim should have this in mind when saying Sheheheyanu at night, and Ashkenazim should have this in mind when reciting the Berakha of Sheheheyanu during the day.
(See Sh. 'A. 692: 1 with Rama. Hilkhoth Haggim Maamar Mordechai 64: 15)
