We have an existential control on relations, which works well for
He can’t swim.
It means the ToSwim relation doesn’t exist for him, so "He swam the Hellespont" must be erroneous.
But "He can’t fail" is different. We are expressing an existential negative on a relation which supplies a logical negative on another relation – it is "fail to do".
He failed to win.
He didn’t fail.
He can’t fail to win.
An existential negative on a relation controlling the propositional logical connection on another relation puts out a true, rather than the relation ceasing to exist.
We could treat this as an existential antonym, where an existential false goes to a logical true:
ToFail ToSucceed
Can’t fail means must succeed
Which brings us to
He neglected his duty.
a
He neglected his duty is "he neglected to do his duty", so
He can’t neglect his duty
There are several relations that have this property:
ToNeglect
ToFail
ToPrevent
We have to take it into account when building them (by adding a relation they can control if it is not explicit) and when changing their existential state.