Is the pronoun 'it' used correctly in this sentence?

The pronoun it in the sentence

A controlling idea: What the writer is going to focus on it in the paragraph.

is unnecessary and ungrammatical.

Such pronouns, which are usually but not always in a relative clause, are called resumptive pronouns. This is a grammatical feature of some languages, but not English.

An exception in English is the informal resumptive pronoun in statements such as following, with the resumptive it:

This virus, it's such a tragedy.

Your sentence should be rewritten without the pronoun as:

A controlling idea: What the writer is going to focus on in the paragraph.

Alternatively:

A controlling idea is what the writer is going to focus on in the paragraph.

Wikipedia has this on resumptive pronouns in English and other languages.