Jack, the more prosaic explanation is that a person previously infected who is not clear of the virus has been erroneously told, after a test, that they are clear. Then, when symptoms recur, they think they've had it twice. As the testing is notoriously unreliable (I read somewhere...can't remember now... that around 30% of tests give false results), it behoves us to be cautious about claiming that immunity is either short-lived or non-existent. For a start, that would be somewhat untypical of this type of virus. There is a lot we don't yet know about this, and it would be best to avoid the kinds of wild conjecture we're seeing in this thread.