I came across the following post in the Guru Q & A -

https://www.bankersonline.com/qa/reg-e-provisional-credit-5-day-reversal

This doesn't quite address another situation that I was interested in finding an answer on. What if the bank makes a final decision on the dispute and issues the notification that the provisional credit has been made final, but subsequently determines the merchant also reimbursed the customer. I have come across some posts about this being "unjust enrichment," but it sounds like you have to file a civil suit to get such moneys back. Is that correct, or is there a way for the bank to just take the monies back? What if the bank gave provisional credit under Reg. E when they didn't have to, such as if it was really just a merchant dispute, rather than a valid Reg. E Error Resolution situation? Then, could a bank take the money back under the pretense that this was a bank error that they are entitled to recoup?

You simply send a letter stating that the customer received a duplicate credit which you are reversing. Don’t reopen the investigation. Don’t revoke provisional or final credit. The customer has money in their account to which they are not entitled.
You are correcting that.

Isn't there a rule against taking back a finalized provisional credit? From everything I came across, it didn't seem to be as simple as just reversing a "duplicate credit." It may not quite be a duplicate credit also.

You are not taking back the provisional credit. Once you make the customer whole, then you step into their shoes as far as any recovery. If the merchant then refunds money, it is yours, not the customers.