The necessary and sufficient conditions of similarity of two idempotent matrices are proved, the idempotency of linear combinations of two idempotent matrices is discussed.
I mean, you could just limit to one request per day, but if you're not going to do that, how do people usually implement or ensure idempotency in their applications?
The HTTP standard has actually a new, has defined a header where you can put an idempotency key and the servers, when they support this, are supposed to understand if that key is set.