In the context of REST APIs, when making multiple identical requests has the same effect as making a single request – then that REST API is called idempotent.
When you design REST APIs, you must realize that API consumers can make mistakes. They can write client code in such a way that there can be duplicate requests as well. These duplicate requests may be unintentional as well as intentional some time (e.g. due to timeout or network issues). You have to design fault-tolerant APIs in such a way that duplicate requests do not leave the system unstable.