Comentario sobre Números 4:30

לעבוד את עבודת אהל מועד, "to perform the service connected with the Tent of Meeting." When describing the parallel task performed by the sons of Gershon the Torah wrote לעבוד עבודה באהל מועד, to perform service inside the Tent of Meeting. The reason is that they had to transport the coverings of the Tabernacle, the curtain, etc., all of which were part of the actual Tabernacle; the sons of Merari, however, transported the outer parts of the Tabernacle, the boards, the bolts, the sockets supporting the boards, etc. This is also why the Torah described the work of the Merarites as לעבד את…הקרשים as this represents the עבודת אהל מועד, i.e. work related to the Tabernacle. The Torah did not fear that by writing only the word לצבא alone instead of לצבא צבא that we would misunderstand its meaning, seeing it had spelled out exactly what the duties of the Merarties were.