The following inference: IF A is responsible for X THEN B is not responsible for X is false, in general. Responsibility does not mutually exclude. It is of

The following inference: IF A is responsible for X THEN B is not responsible for X is false, in general. Responsibility does not mutually exclude. It is often arranged that responsibilities do not overlap, but this is an organisational expediency. It is not a moral principle. So saying "the responsibility was his" as if that somehow means that no-one else can have any responsibility is wrong.