The situation begs these questions:
How are these customers "getting" these redemption codes?
Are the codes already "doubled" in the email they received?
Are they sent from "you" via your store Admin area?
Are they getting it from other customers sharing GV's with them?
Are they purchasing these GV's and the store is emailing it to them?
Do you have the admin GV Queue enabled? If so then it could be something in the queue approval process.


(GV = "Gift Voucher/Certificate")