に～を重ねて【に～をかさねて】

に〜～をかさねて重ねて puts the same noun on both sides of に and を: どりょく努力にどりょく努力をかさねて重ねて, "effort piled on effort". It says that something was done over and over, and the sentence goes on to what all that repetition finally brought about.

The noun is something you do, usually with effort: どりょく努力, けんきゅう研究, れんしゅう練習, ぎろん議論. The pattern is formal and belongs to writing, speeches and the news. In conversation, say なんど何度も with the verb.

を重ねて, without the doubling

With one noun, をかさねて重ねて says the thing built up over time. Doubling the noun stresses that it went on and on, often through hardship.