This word كَرَّتَينِ is in the dual form of once so it means twice. The perfection of the heavens will return your eyesight humbled. Why is looking at the heavens so important? Why is it being repeated over and over? Because looking at the signs from Allah and pondering over the creation of the heavens and the earth can be used as a means to save us from the fire.

الَّذِينَ يَذْكُرُونَ اللَّهَ قِيَامًا وَقُعُودًا وَعَلَىٰ جُنُوبِهِمْ وَيَتَفَكَّرُونَ فِي خَلْقِ السَّمَاوَاتِ وَالْأَرْضِ رَبَّنَا مَا خَلَقْتَ هَٰذَا بَاطِلًا سُبْحَانَكَ فَقِنَا عَذَابَ النَّارِ

“Who remember Allah while standing or sitting or [lying] on their sides and give thought to the creation of the heavens and the earth, [saying], “Our Lord, You did not create this aimlessly; exalted are You [above such a thing]; then protect us from the punishment of the Fire.” (Surah Ali ‘Imran: 191)

The purpose of looking and pondering over the skies is to strengthen and to make our faith firm. When you feel your Iman going down, look at the sky; try to grasp its flawlessness, beauty, and how magnificent and endless it is.

There are no cracks, tears or deficiencies in the sky. Then look again, and yet again.