Malbim: One teaches us that Hashem Himself counted them, which explains why no half-Shekel was needed, and why the counting caused no harm, even though even babies were counted; the other teaches us their numbers.

Oznayim la'Torah: It inserts "Pekudeihem" twice by the B'nei Gershon, based on the principle that 'One Ribuy (an inclusion) after another denotes a Miy'ut (a preclusion)', and Gershon had decreased from two families to one. ¹