I know it's wrong, but I like to go by the first answer in the thread. Each effect in unique, and you must save for each, even if they don't stack. This and that same attacks doesn't stack also.
So, if you get "Ongoing 5 Fire", and get it again from the same attack, you still got only one. But if you get "Ongoing 5 Fire" for another, you must save twice to get rid of it.
The principal point is, to me, that save ends is a duration. If you make the save, the duration ends early, but if you fail, it ends later. So, the longest duration wins.