Idempotency

Means a function can be composed with itself and form the same result.

It doesn't necessarily mean you can do the same thing multiple times without changing a result.

Is this the technical definition?

(= (comp f f) f)

(inc 1)
(inc 1) ;; same result!

(inc (inc 1))

So inc is not idempotent.

Example

Side effects

Elevator buttons are idempotent. You can keep pushing button 5, and the elevator won't change anything. It will still go to floor 5.

PUT vs POST. If I POST the same thing twice, will there be two of them, or one of them? The HTTP spec encourages PUT to be idempotent and POST not to be. So, if I PUT two of the same thing, there should only be 1 saved. If I POST 2 of of the same thing, there will be 2 of them.