ואלה תולדות אהרן ומשה, first the Torah counted the descendants of the people at large, followed by that of the priests, and finally the Levites. Now the Torah enumerates the personal offspring of Aaron. The offspring of Moses is lumped together with that of the Levites, as we know from verse 27 “and as for Kehat, the family of Amram, the family of Yitzhar, etc., etc.” Amram had only two sons, Aaron and Moses. Both were included in the list of the members of the tribe of Levi. Aaron’s sons had the additional distinction of being sanctified.

ביום דבר ה' את משה בהר סיני, these words are added as the conversation occurred before the Tabernacle had been erected when Aaron still had four sons as opposed to the second year (mentioned in 1,1) when the Tabernacle had already been erected and Aaron had only two sons left, Eleazar and Ittamar, as I have explained already at the beginning of our portion. Once the Tabernacle had been erected, G’d is no longer referred to as speaking to Moses at Mount Sinai.

נתונים,נתונים, the repetition of the word is similar to the repetition נתן תתן להם אחזת נחלה, “be sure to give” in English, in Numbers 27,7.

ואת אהרן ואת בניו תפקד, you shall appoint Aaron and his sons to be in charge of the Levites, as in the word ויפקד in Genesis 40,4 where the warden of the jail appointed Joseph to be in charge of all the prisoners.

כל זכר מבן חדש ומעלה תפקדם. Seeing that they needed to redeem the firstborn of the Israelites at large whose redemption must take place when they are one month old, the Levites also had to be counted already from that age onward.

ולקחת משפחת העמרמי, a reference to Moses and his sons. Now the Torah spells out what had been meant in verse one by the words ואלה תולדות אהרן ומשה.

הפדיום. The final letter ם here is superfluous just as it is superfluous in the word ריקם instead of simply ריק. (Genesis 31,42), or the same letter in the word הכנם in Exodus 8,13, or in the word שלשום in Genesis 31,2. Our sages explain that there were a total of 300 Levites in excess of the number of firstborn that they were to redeem (replace). (Bechorot 5, compare Rashi on verse 39). The reason that the verse described these as “leftover,” “in excess,” is that a firstborn cannot redeem another firstborn, i.e. there must have been a number of firstborn Levites also.