«فَلَا تَخْشَوُا النَّاسَ وَاخْشَوْنِ»
“So do not fear people; fear Me.”
The common root of fear appears first under prohibition and then as a command. This contrast of denial and affirmation forms negative ṭibāq.
It also directs the reader’s attention from one object of fear to another. The polarity is tied to a change in the proper orientation of the action, not a decorative repetition alone.