Try updating to the recommended PHP 7.4 maybe...
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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")
Hi DrByte! Thank you for your answer!
I send the GV from the Admin panel, and they receive the email with the code sent from the store. Once they click on the link to redeem the GV and return to the store, the code appears duplicated on the store page where it is redeemed, and they cannot redeem it, receiving this message (the number is just a sample): "The Gift Certificate number fbdc184810fbdc184810 may be invalid or has already been redeemed. To contact the shop owner please use the Contact Page." Yes, the admin GV Queue is enabled.
I hope this helps to clarify the issue.