The article a placed in front of a noun conveys the idea of one. When writing about two separate objects, a second a must be used. If the two objects are considered as one, then the second a may be left out.
Incorrect: I have a hammer and chisel.
Correct: I have a hammer and a chisel. (Here we are talking about two separate objects.)
Incorrect: She has a son and daughter.
Correct: She has a son and a daughter.
Incorrect: The mechanic used a block and a tackle to lift the machine.
Correct: The mechanic used a block and tackle to lift the machine. (Here we are talking about one object.)
