Here is an example that is on point: https://ferenczylaw.com/solutions-in-a-flash-overpaymentsif-it-comes-back-it-is-yours/
Essentially, the individuals who received an overpayment of a refund are given the opportunity to return the amount of the overpayment to the plan.
While not explored in the example, the timing of an repayment could have an impact on issues like lost earnings. If the overpayment already was reported as taxable income on a 1099 then there could be issues of correcting the 1099, or creating basis in the account.
Document everything, and keep that documentation to just this side of forever.