Exactly-once is only meaningfully possible if you have a rollback for tasks of unknown completion state - for example if the task involves manipulating the same database as the one controlling the task execution. Otherwise, it becomes the (impossible to solve) two-generals problem between updating the task status and performing the task.
There is actually another possibility: there must be a way to check whether the receiving system has received the message. But this only works if there are no "rogue" senders.
