What kind of grammar is it in 調査に調査を重ねて?


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?




> 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.




> 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.

