Question

クーリッシュ

下線部でhadを2回使えるのはなぜですか

However, it was not always convenient or possible to be on guard at these times.
15 Mr. Flowers had tried in many ways to prevent damage to his glass; but (B)nothing
that he had done had been successful. He had been to the school to complain to the
headmaster; but this had not done any good. He had hidden in bushes and chased
1. that thuomatoncs into his cordon but the hous could run faster than he could and

Answer 1

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ゲスト

nothing (that he had done) had been successful

nothing が先行詞で、関係代名詞節(that he had done) が修飾しています。

nothing (～) had been successful
というSV が見えます

関係代名詞節内で過去完了を作るhad
主節のhad been(過去完了)
それぞれ過去完了を作るhad ですから問題ありません。