Durable PHP has a way to guarantee exactly-once side effects through "activities" (I assume you are talking about the outside world side-effects, not internally). Trying to do the same activity twice will simply result in merging with the previous execution. This isn't integrated with actors -- mostly because I hadn't thought about it before. It wouldn't be hard to do though.

However, in most async systems, you can only guarantee at-most-once or at-least-once; exactly-once is quite hard (and why there is a dedicated way to do it in this framework).

Internally, messaging is set up to guarantee at-least-once and duplicate messages are ignored (as well as dedicated ways to ensure deterministic execution).