Authentic Hadeeths from the Prophet Muhammad (SAWA) explaining the meanings of Sura Al-Rahman; verses 19 to 22, مَرَجَ البَحرَين يَلتَقِيان (Allah Has created two seas which merge together) stated that the two seas are Ali and Fatima who merged and from them jewels came out who are Hasan and Husain. This Hadeeth has been narrated our authentic books and in Sunni books like:
1. Yanabee' Al-Mawaddah by al-Qondoozi Al-Hanafi from Abi Sa'ee Al-Khudri and Abdullah Ibn Abbas and Anas Ibn Malik.
2. Al-Kashf WAL Bayan by al-Tha'labi from Sufyan al-Thawri.
3. Shawahud al-Tanzeel by al-Hasakani.
4. Rooh al-Ma'ani by al-Aaloosi al-Shafi'ee.
5. Maqtal al-Husain by al-Khawarizmi al-Hanafi.
6. Al-Durrell al-Manthour by al-Soyoti al-Shafi'ee.
7. Al-Manaqib by al-Kashfi al-Hanafi.
8. Kitab al-Shifaa' by Al-Qadhi 'Eyadh>
9. Tathkirat Al-Khawaass by Sibt Ibn al-Jawzi al-Hanbali.
10. Nozhat al-Majaalis by al-Safoori al-Shafi'ee.
11. Miftah al-Najaa by al-Badakhshi.
12. Al-Haafidh Abu Na'eem in his book; Maa Nazal min al-Quran.
13.Ibn al-Maghazili in al-Manaqib.
14. Ibn al-Sabbagh al-Maliki in al-Fosool al-Mohimmah.
15. Al-Balkhi al-Shafi'ee in al-Manaqib.
I one saw someone teaching Surah al-Rahman by associating meaningful signs with each of the verses, so, for instance, in this verse, they pointed to stars and made trees with their hands and fingers. The idea behind this is that the human being will remember gestures and words more than gestures. You could try it!
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.