Idempotency
An operation is idempotent if running it twice has the same effect as running it once. It's what stops a retried automation from creating duplicate records, sending a message twice or charging a customer twice.
Retries are unavoidable — networks time out, APIs return errors, webhooks get delivered more than once. The question is whether a retry is safe. "Create a customer" is not idempotent by default; "create a customer with this specific ID, if one doesn't already exist" is.
In practice this means giving each operation a key derived from the event, and checking it before acting. It is the single most common thing missing from automations built quickly, and it shows up as duplicate invoices and double-sent emails weeks later.
Why it matters
If an automation has ever created the same record twice, this is why. It's cheap to build in and awkward to retrofit.
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