What is the significance of the two commands, one here, and one in Devarim 6:1?
Ramban: The former refers to the Aseres ha'Dibros, which they heard directly from Hashem; the latter, to the rest of the Torah, which following their request, with which Hashem agreed, they heard from Moshe Rabeinu. 1
Seforno: The former refers to the Mitzvos that applied to them until now; the latter, to the Mitzvos that were connected to Eretz Yisrael, which would take effect once they entered Eretz Yisrael.
Ramban: As the Pasuk specifically states.
And what the connotations of "ve'Lo Sasuru Y'min u'Semol!"?
Seforno: It means that they should not add to the Mitzvos or subtract from them.