Mastercard Reason Code 4834

The cardholder was charged twice for one purchase, or paid through two different methods.

A retry, double-submit, or multi-channel payment resulted in the same order being paid for more than once.

If a genuine duplicate occurred, refund it directly rather than contesting; otherwise your order log is your defense.

Order/payment log showing only one legitimate charge was intended for this purchase.

No idempotency protection on checkout submission, letting double-clicks create duplicate charges.