Then, it assumes that your actors don't have any side effects. (Otherwise, "retrying" is not an option.)
I found that surprisingly hard in most applications - and I think that's is a limitation of the actor model _in practice_, that is often overlook because it's only an implementation detail that does not really exists _in theory_.
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).
I found that surprisingly hard in most applications - and I think that's is a limitation of the actor model _in practice_, that is often overlook because it's only an implementation detail that does not really exists _in theory_.