Dickens is famous as . . . . . . . . novelist and . . . . . . . . essayist.

In the first blank, 'novelist' is general class of people and indefinite article be used. As 'novelist' begins with consonant sound, article 'a' will be used.
In the second blank, no article will be used because 'novelist' and 'essayist' is the same person and article will be used only once.