Or HaChaim on Numbers 4:23/אור החיים על במדבר ד׳:כ״ג

1 תפקד אותם, "you will count them." Why did the Torah have to repeat the instruction to count seeing the word נשא already meant "to count?" I have explained this in connection with the instruction to count the Kehatites (4,20).

2לצבא צבא, "to perform public service etc." When speaking of the service to be rendered by the Kehatites the Torah described this only as לצבא, without the word צבא. This was because they carried only the holy vessels that formed the interior furnishings of the Tabernacle. The materials carried by the clan of Gershon were only vessels for the vessels so to speak, i.e. לצבא צבא. [The author perceives the furnishings of the Tabernacle as being the "army" of the Tabernacle, so to speak. Ed.]