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

Question

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?

Answer 1

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.

Answer 2

「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".