[NEW EXAMPLE] An example about how to write idempotent functions

[Nsupyq]

Idempotence is an important property to perform fault tolerance. However, I find this repo lacks examples about what is idempotent functions and how to write idempotent functions. Therefore, I think it is necessary to contribute such examples. As suggested in PR #658, I have created new examples about idempotence based on existing examples in this repository.