«فَبِأَيِّ آلَاءِ رَبِّكُمَا تُكَذِّبَانِ»
Which of your Lord's favors will you both deny?
Ayyi is an interrogative noun in an iḍāfah construction with ālāʾ; tukadhdhibān is a dual present verb. Its form is a question, yet its contextual force includes recalling blessings and reproaching denial.
The recurring verse is not purposeless repetition. In each new setting, its recurrence reasserts the significance of the favors or signs just mentioned. Rhetorical repetition must be evaluated in context.