Deduct Points on a Payment-Completed Event

This challenge has you build a NestJS consumer that deducts user points from a payment-completed event. The payment system delivers the message at least once, and the same message may arrive again depending on network conditions. You complete the consumer's handling of the payment-completed event and use the in-memory store as the single source of truth for balances.

This problem is about handling where "the same message arriving again must be reflected only once." For resources where something that happened once must be applied only once — points, balances, inventory — code can look fine when the message arrives a single time, yet the real test is the moment the same message is redelivered. Verifying code like this means going beyond a single-process check: you feed the same message twice and confirm the result is reflected only once.

Why isn't a single-process test enough for event-handling code like payment completion?

When you feed the message only once there is no further message to arrive, so it looks fine in normal use. The issue only shows up the moment the same message is redelivered, which you confirm by feeding the same event twice.

How do you verify handling that withstands redelivery?

Process an event for the same order twice, then assert that the balance dropped by only one round's worth. The result must match what you get when you process it only once.