How did the double billing bug work? What is its logic and mechanics?

A double deduction bug occurred when resources (such as in-game currency or items) were deducted twice due to synchronization errors or repeated request submissions.

The bug's logic was that after the first deduction, the system did not correctly update the state or block the repeated operation, so when the deduction was called again, it happened once more, leading to an incorrect reduction of the player's balance.

The mechanic could be related to the lack of idempotency of operations or improper handling of transaction confirmations. For example, if the client resubmitted the request due to a timeout, the server did not recognize that the operation had already been completed and deducted the funds again.