In
duplicative causation, each cause is a factual cause.
You know
the duplicative causation (overdetermined injury) pollution scenario.
OK, but in
your duplicative causation cases, I don't quite understand how you are using «factual» causation.
Paras. 39 and 46 (2) as written literally cover
duplicative causation too.
In cases of
duplicative causation, one can, obviously, say that at least one of the tortious causes was necessary for the injury — because everything has to have a cause — but that isn't how «necessary» is used in but - for.
Cook v Lewis was not
duplicative causation.