How to remove duplicate URL from Google Search? (indexed all *.html urls by accident)

You should have a canonical URL on each of your pages. Read here: 網址標準化和標準標記 | Google 搜尋中心  |  說明文件  |  Google Developers. Then, even if Google sees duplicate pages, it won’t be a problem.

As for what you can do for now is, try to remove the duplicate pages from Search Console.

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