Where Duplicates Come From

On the morning of day 12, a customer writes to the support team of an online store. She bought one item on order O-7731, which earns 150 loyalty points, and her account shows 300. The support engineer finds two PointsEarned events for the same order in the purchase-events topic, at offsets 0 and 1, and both carry the same event ID.

That same week, the points service had been restarted during a deployment, and about 250 other customers were credited twice. Nobody had connected the two problems, because both looked like "the points service counted something twice."