فَارْجِعِ الْبَصَرَ هَلْ تَرَىٰ مِنْ فُطُورٍ
Look again: do you see any cracks?
Al-Mulk 67:3 speaks of the created heavens and absence of disparity, then commands the observer to look and asks whether a flaw is seen. Verse 67:4 calls for renewed inspection.
The repetition supports the argument by challenging the observer to examine again. The interrogative is not simply a request for a numerical report, and thumma links the renewed act of looking to the first.