Idempotency
The property that repeating the same request several times leaves the same result as doing it once.
Glossary · Maintenance & technical quality
On a network, requests sometimes break off: the sender gets no reply and does not know whether the action was carried out. The safe thing is to repeat it, but only if the repeat will not create a second order or a second payment.
An idempotent operation guarantees that. Usually each request is given a unique key: the receiver remembers it and, on getting the same key again, returns the earlier result instead of performing the action anew.
Without this property, integrations create duplicates, such as invoices issued twice or doubled orders, and it comes to light only when the data is reconciled. It is therefore worth asking about repeats while an integration is still being designed.