A relative clause usually begins with a relative pronoun or a relative adverb. When relative pronouns (who, whom, whose, which, that) is the object of a verb or preposition, we never use another word with the same function later in the clause.
The dog ran after the car that we wer riding in.
(Not: The dog ran after the car that we were riding in it)
Similarly, when a relative adverb (when, where, why) begins a clause, another adverb must not repeat its function:
The place where I work is five miles from my apartment
(Not: The place where I work there is five miles from my apartment)
We took them to a retaurant when they came to visit.
(Not: We took them to a restaurant when they came to visit then)
A relative pronoun in a defining clause can be omitted when it is the object of a verb or preposition, but not when it is the subject of a verb.
My dad still owns the car (that) he bught in high school.
The suit (which) he was married in was his cousin’s.
The people who own this house must be rich
(Not: The people own this house mus be rich)
Relative adverbs are never objects; however, a relative adverb inth middle (rather than the beginning) of a sentence can often be omitted:
The office is the place (where) you spend most of your life.
There are times (when) I regret my decision.
Even in sentences without a relative clause, English usually does not allow a noun function to be repeated by a pronoun. This is so even when a word group comes between the subject and verb.
Life is beautiful.
(Not: Life, it is beautiful)
The doctor advised me to quit smoking.
(Not: The doctor he advised me to quit smoking)