Idempotency only refers to state changes from subsequent invocations of the call. A command to add a user to the set of users that have upvoted a post would be

Idempotency only refers to state changes from subsequent invocations of the call. A command to add a user to the set of users that have upvoted a post would be idempotent. Because you can run it 20x and only the first call affects anything. A command to increase the upvote count for a comment by +1 would not be idempotent.