If a fact has been published, does that mean that it can then be republished even if it is damaging to national security?

That depends on how and where the information has been published.

Sensitive information which has already ‘been widely disclosed or discussed’ should not be withheld.

However judgements about what is ‘widely available in the public domain’ tend to be subjective.  A statement by a writer with no authority in an obscure publication, broadcaster or website is unlikely to have a significant impact or effect.  Therefore, each case has to be assessed on its own merits.