Glossary
A operation is idempotent when running the same request several times has the same result as running it once.
In distributed systems the retry is the normal case: a call times out, the caller does not know whether it arrived, and tries again. Without idempotency that becomes a second payment or a duplicate order.
It is usually implemented with a key supplied per operation, which the receiver stores. If the same key arrives again, it returns the stored result instead of processing again.