A common error is placing a pronoun immediately after a noun subject, as in 'My friend, he is coming' or 'The students, they finished early.' In Standard English, once you have named the subject with a noun, you do not repeat it with a pronoun. The correct forms are 'My friend is coming' or 'The students finished early.' This error often appears in spoken English or informal writing but should be avoided in formal contexts.
You already know that pronouns substitute for nouns — instead of repeating "my friend" in every clause, you say "he" or "she." And from subject-and-predicate, you know that the subject is the noun phrase that the sentence is about. The double-subject error happens when writers accidentally combine both: they state the subject as a noun, then immediately restate it as a pronoun before giving the predicate. "My sister, she is a doctor" has two items in the subject position — the noun phrase "my sister" and the pronoun "she." Standard English requires one or the other, not both.
Think of it as a pronoun doing its job too eagerly. The pronoun exists to substitute for a noun that has already been introduced — usually across sentences or across a gap in discourse. "My sister is a doctor. She works at City Hospital." That is the correct pattern: noun first, pronoun for the next reference. The double-subject error collapses this across-sentence pattern into a single sentence, placing the substitute and the original side by side in the same subject slot. It creates redundancy because the noun and the pronoun refer to the identical entity.
The error is common in speech, particularly in languages where resumptive pronouns — pronouns that "resume" a displaced subject — are grammatically standard. In some languages, a structure equivalent to "My friend, he called" is not only acceptable but required in formal writing. Speakers of those languages applying their native grammar to English produce this pattern naturally. In Standard English, however, the subject noun phrase takes the verb directly, with no resumptive pronoun: "My friend called," not "My friend, he called."
Correcting the error is simple once you see it: remove the pronoun and let the noun phrase carry the subject alone, or remove the noun phrase and use the pronoun. "The students, they finished early" becomes either "The students finished early" or (if context makes the referent clear) "They finished early." In formal writing, use the full noun phrase unless you have already introduced the referent in a prior sentence and are continuing a chain of reference.