Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

Add words needed to complete compound structures
Add needed words
Clarity

Sometimes writers leave out words without affecting the meaning of the sentence. But often the result is confusing or ungrammatical. Readers need to see at a glance how the parts of a sentence are connected.

FOR MULTILINGUAL WRITERS

Languages sometimes differ in the need for certain words. In particular, be alert for missing articles, verbs, subjects, or expletives. See 29, 30a, and 30b.

10a Add words needed to complete compound structures.

In compound structures, words are often left out for economy: Horatio is a man who means what he says and [who] says what he means. Such omissions are acceptable as long as the omitted words are common to both parts of the compound structure.

If omitting a word from a sentence would make the sentence ungrammatical because the word is not common to both parts of the compound structure, the word must be left in.