Let's use comic book fans for example. There's this group of comic book fans who pretty much know everything there is to know as they discuss things daily. There's new information of a new comic coming out, and so an outside person who doesn't really care about comics comes in to share the news like it's something exclusive, but the comic book group already knows about it.
Is there an idiom or term to describe what the outside person is doing? Attention seeker? It's kind of like stating the obvious.