Duplicate subject is a common mistake made by ESL students. They sometimes use a pronoun as a duplicate subject in a clause that already has a noun as a subject.
Remember that each clause can have only one subject. If you have already mentioned the subject by name, it is wrong to use a pronoun to refer to the same person.
My sister is a nurse. OR She is a nurse. (BUT NOT My sister she is a nurse.)
Another common mistake is to use two subjects in a relative clause. Note that the relative pronoun serves two purposes. It connects two clauses like a conjunction. It also serves as the subject or object of the verb in the relative clauses. As a result, it is wrong to use another noun or pronoun as the subject or object of the verb in the relative clause.
Study the examples given below.
My sister is a nurse. She lives in California.
My sister, who is a nurse, lives in California. OR My sister, who lives in California, is a nurse. (BUT NOT My sister, who she lives in California, is a nurse.)
Here the relative pronoun who stands for the pronoun she and hence it is wrong to repeat the pronoun in the relative clause.
Another example is given below.
Maya does my hair. She has moved to another hairdresser’s.
Maya, who does my hair, has moved to another hairdresser’s. (BUT NOT Maya, who she does my hair, has moved to another hairdresser’s.)