If my observations are correct, the behaviour of "Notify all who received any messages regarding the problem before" is not documented as good as it could be:

If you combine this operation type with "Send recovery message" in an action, I would expect a potential overlap between the two, if I specified the same users again. If I'm getting the code right, the SQL where clause in add_sentusers_msg() has both, the problem and the recovery eventid to consider, which results in this de-duplication. This is obviously on purpose. However, I would suggest to state this clearly in the documentation!