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I did not find any resources explaining the following grammar.

> N に N を V

For example,

> **調査に調査を重ねて** 事故の原因を究明した。

What kind of grammar is it? What does it mean?
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2016-12-13T15:48:54.790
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2016-12-14T01:13:02.087
2016-12-14T01:13:02.087
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question
grammar set-phrases
What kind of grammar is it in 調査に調査を重ねて?
> X に X を重ねる (lit. _to pile up X (further) onto X_ )

is an idiomatic phrase to mean "doing X extensively, or unthinkably many times
over", usually X is a noun that means some kind of (hard) work.

I also often hear:

> 研究に研究を重ねる  
>  努力に努力を重ねる  
>  改良に改良を重ねる etc.
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2016-12-13T16:07:29.623
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2016-12-13T16:07:29.623
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answer
> 「N に N を V」 when the verb is 「重{かさ}ねる」

means:

> "to do/make one N after another" or
>
> "to do/make N repeatedly"

「調査{ちょうさ}に調査を重ねる」 thus means " _ **to conduct one investigation after
another**_ ".
CC BY-SA 3.0
2016-12-13T16:11:52.950
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2016-12-13T16:11:52.950
2020-06-17T08:18:27.500
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answer