Three idiomatic expressions may be relevant to your situation.
First late to the party is sometimes used figuratively to suggest that the person offering the information has come with it behind times—too late for the information to be news to the person or persons addressed.
Second, stop the presses originated as a cry used in the newspaper trade on the rare occasions when an edition of the paper is on the press and being printed but important new information has just come in that renders the current story on the subject obsolete. The expression is sometimes used sarcastically to suggest the very opposite situation—namely, that everyone already knows the volunteered information or that it is too trivial to affect the existing view of the situation.
Third, old hat means, according to Idiom Corner, "not new or different, old-fashioned." It, too, can be used in the figurative sense of "behind the times and therefore not news."