There is no question of any word in Quran mentioned just for poetic reason. Every word in Quran has its important meaning even if it was repeated in previous verses, but the meaning of it in every verse has its own situation which is different from other verses.
In Sura Al-Rahman, the verse (Fabi'ayyi Alaa'i Rabbikoma Tokaththibaan) فبأي الاء ربكما تكذبان was mentioned thirty one times, but in every time, the meaning of it is related to the immediate verse before it. Allah (SWT) in Sura Al-Rahmaan is mentioning His bounties, then asking Jinns and humans : Which bounty of your Lord you deny? This question is repeated after every bounty because every bounty is different from other bounties.