يَوْمَئِذٍ يَصْدُرُ النَّاسُ أَشْتَاتًا لِيُرَوْا أَعْمَالَهُمْ
That Day people will emerge in separate groups to be shown their deeds. (99:6)
Ashtātan serves as a circumstantial accusative, indicating the state in which people emerge. The lām in li-yuraw expresses purpose, and the verb is passive: the point is that their deeds will be made visible to them. Reporting by the earth and disclosure to people now belong to the same scene.
Repeating yawmāʾidhin situates successive events on a single decisive day. Its value is temporal linkage and renewed focus, not necessarily a bare repetition with no further function.