Opuszczanie "had"

Temat przeniesiony do archwium.
Fragment książki:

The stairway was dark and narrow, and "back" stairback, but he had known and made a study of all that before...

Fragment zadania z "Grammar in context"

Brenda pearl joined our firm ten years ago. She had worked for the previous five years with an advertising company and had acquired much useful expierence.

Jak to jest z opuszczaniem "had" w drugim "clause"? (W kluczu do zadania z "Grammar in context" nie ma w kluczu opcji opuszczenia "had")
Someone has voiced their opinion on the subject:
https://ell.stackexchange.com/questions/118350/repeating-have-or-other-auxiliary-verb-in-one-sentence
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