Tafsir of At-Tur 52:16

This means: Enter it and endure its severities, then do whatever you wish regarding endurance or the lack thereof.

Sawā’un ‘alaykum (It is the same for you): Both matters are equal for you in their lack of benefit, since neither will repel the punishment nor alleviate it. Sawā’un is the predicate of an omitted subject; it is valid to predicate it of the dual [the two matters] because it is originally an infinitive. It is also permissible that it is a subject whose predicate is omitted, but that is not as strong.

As for His saying, the Exalted: Innamā tujzawna mā kuntum ta’malūn (You are only being recompensed for what you used to do):

This is the justification for the equality. Since the recompense is bound to occur due to the preceding warning and His—glory be to Him—decree of it according to the requirements of His justice, endurance and the lack thereof are equal in their lack of benefit.