Tafsir of Al-Qamar 54:21

This is a magnification of them both, and an expression of astonishment regarding their state after having explained them. Thus, there is no trace of repetition in it in relation to what has preceded.

It has been said: The first [mention] refers to what befell them in this world, while the second refers to what will befall them in the Hereafter. The use of "was" (kāna) is for the purpose of parallelism (mushākalah), or to indicate its realization, following His custom, Glorified is He, in His informing.

This is countered by the fact that the second [statement] follows the worldly punishment.