Full Idea
In intuitionist logic, if we do not know that we do not know A, it does not follow that we know A, so the inference (and, in general, double negation elimination) is not intuitionistically valid.
Gist of Idea
Double negation elimination is not valid in intuitionist logic
Source
Michèle Friend (Introducing the Philosophy of Mathematics [2007], 5.2)
Book Reference
Friend,Michèle: 'Introducing the Philosophy of Mathematics' [Acumen 2007], p.107
A Reaction
That inference had better not be valid in any logic! I am unaware of not knowing the birthday of someone I have never heard of. Propositional attitudes such as 'know' are notoriously difficult to explain in formal logic.