TT #32 · Parashas Va’eschanan · Passuk 6:9
And you shall write them on the doorposts of your house and on your gates.
Torah Temimah points to Menachos 44a (Rav Sheshes: one who has no mezuzah on his doorway violates two positive commandments).
English: Rav Sheshes said: Anyone who has no mezuzah on his doorway violates two positive commandments — “and you shall write them” (here) and “and you shall write them” (in Parashas Eikev).
Diyuk of the repeated command “u-chesavtam” in two places as two separate asei.
Absence of a mezuzah constitutes the violation of two independent positive commandments.