Why does the Torah repeat the word "Nesunim"?
Rashi: Because the Leviyim were designated for Hashem as regards carrying and as regards singing. 1
Seforno: The first "Nesunim" refers to their volunteering to serve Hashem; 2 the second, to the fact that they were given to Hashem from the rest of Yisrael, who became obligated to sustain them with Ma'aser Rishon.
Why does Rashi omit guarding the gates, which is perhaps hinted in Pasuk 19, where the Torah writes "Nesunim" a third time - particularly as there, it writes "Nesunim l'Aharon ul'Vanav", and the Leviyim did indeed help the Kohanim to guard? Refer also to 8:19:2:1.