Tosfos and the Ran hold that the primary kri'as hamegillah is during the day, as reflected in the gemara's (daf 4) limud from the pasuk "ekra yomam...v'layla lo dumya, which places "yom" first. Tosfos rules therefore that even though the bracha of shehechiyanu is recited on k'rias hamegillah at night, it must be repeated during the morning kri'ah as well when the primary pirsumei nisa takes place. The Rambam (1:3) however rules that the bracha of she'hechiyanu is not repeated.
This machlokes can be understood in one of two ways: 1) the Rambam argues with Tosfos's fundemental point and holds that the kri'ah of day is no more significant than the kri'ah of megillah at night; 2) the Rambam agrees with Tosfos's fundemental point regarding the primacy of kri'ah during the day, but holds that two brachos of she'hechiyanu may not be recited over the same mitzva.
This issue is raised by the Aruch HaShulchan with respect to the question of which megillah reading one should aim for - day or night - if one has the opportunity to attend only one of the two.
Perhaps there is a third way to understand the machlokes. Tosfos writes that the kri'ah of day is primary because "ikkar pirsumei nissa" is by day. Kri'as hemegillah has two aspects to it: 1) the mitzva of kri'ah 2) a kiyum of pirsumei nisa. Perhaps Tosfos and the Rambam both agree that one cannot recite she'hechiyanu twice over the same mitzva, and with repect only to the kri'ah night and day are equal. The chiddush of Tosfos is that only the kriah of the day reflects this additional kiyum of pirsumei nisa, and it is the additional kiyum of pirsumei nisa which is now mechayeiv the bracha of she'hechiyanu. The Rambam, however, holds that since the ma'aseh mitzva of kriah is exactly the same as was done the night before, not additional she'hechiyanu should be said.
(As an aside: the gemara has a hava amina that there is no chiyuv of kriya at all during the day, but rather a chiyuv of "shinuy", to learn the halachos of the day. This perhaps indicates that the kri'ah is primarily a kiyum of pirsumei nisa and not an end in itself, otherwise what justification could there be for substituting learning for leining?)