Players can now repeat turns of the Final Coil of Bahamut prior to the weekly reset.

If your party contains players replaying a turn prior to the weekly reset, the number of treasure coffers appearing upon completion will decrease.*

Furthermore, players will be ineligible for rewards from treasure coffers if they replay a turn prior to the weekly reset.

* If there are one to four players in your party replaying a turn, only a single treasure coffer will appear.
If there are five to seven players in your party replaying a turn, no treasure coffers will appear.

Prior to entering the Final Coil of Bahamut, a dialog prompt will indicate both the number of players in your party who have completed the turn, and the number of treasure coffers available upon completion.
Why not allow just one chest to drop and not let the previous people who won it not to roll on it? That seems like the same reward either way.