Does 一週間二回 mean "once a week" or "twice a week"?
I've heard that "once a week" is, 一週間二回, so how would you say "twice a week"?

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Answer

「期間（period）＋particle「に」＋　number +　回（かい counter suffix）」

ex)　一週間（いっしゅうかん　one week=period ）
       一週間に二回　one week(period) + 「に」　＋　２(twice ) 回(times) = twice a week
　　※一週間二回 is abbreviated.(に)　In casual conversations,you can use this expression.

一か月（いっかげつ　one month= period）
       一か月に二回＝twice a month
　　※一か月二回is also OK.

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You usually say "週に１回" for "once a week" and "週に２回" for "twice a week".  〔Other than that, you usually use Arabic numerals, not Chinese ones, when you write Japanese sentences horizontally.〕

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twice a week.

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一週間二回：TWICE a week while 一週間一回：ONCE a week. Hope I could help you.