Cancellation signals are tricky. You of course cannot be sure that the remote end receives it. This turns into the two generals problem.
Yes, you need monitoring for this case. I work on scientific workloads which can completely consume CPU resources. This failure scenario is quite real.
Not all tasks are idempotent, but it sounds like a prudent user should try to design things that way, since your system has “at least once” execution of tasks, as opposed to “at most once.” Despite any marketing claims, “exactly once” is not generally possible.
Good docs on this point are important, as is configurability for cases when “at most once” is preferable.