The code requires at least one lighting outlet - in addition to a separate requirement for a receptacle every 12 ft. along the walls. There's the key: in addition.
Now, a duplex receptacle is just that: two receptacles. If the code requires one every 12 ft., and we install a duplex, then the other half of that duplex can be used to meet the additional requirement for a lighting outlet.
Hair splitting? Perhaps ... but that's a natural result of trying to meet "minimum" requirements.
