Which spouse should claim a UCCB repayment?

That sounds kind of wacky, but basically If a recalculation shows that you were overpaid UCCB, the CRA will send you a notice with a remittance voucher attached to inform you of the balance owing. As the OP was created under your spouse, it should be indicated on her return. Continue to claim the actual benefit under your name since it was the CRA that changed it to begin with.