Is this sentence grammatically valid or not?
People who achieve they do this with perseverance.
One of the two answers to the earlier question Unnecessary pronouns: “The President he issued…” says clearly it's not "correct grammatically", as it is redundant. The other says it is grammatical, [but it's] missing a comma [before the pronoun].
The first of those two earlier answers has only a single upvote, and the second has none at all, so I don't know which to believe.