Is there an Idiom or term for someone sharing information to a group that already knows it?

Question

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.

Anton · Answer

They are teaching their grandmother to suck eggs

= to give advice to someone about a subject that they already know more about than you
Cambridge

Perhaps its meaning is getting lost in time as few people nowadays literally suck eggs. Many years ago people would suck out the egg contents by piercing the egg at both ends and then sucking on one of the ends. You could reverse the procedure and blow out the contents also. It was such a commonplace procedure then that to "teach your grandmother to suck eggs" was like a child trying to teach as new something the grandmother well knew how to do. The saying still survives despite the fine art dying out in our "civilized" and salmonella fearing culture.
English Stack Exchange

You may also like the relevance of "Queen Anne is dead", an idiom discussed well in:

the death of Queen Anne was officially hushed up for a while [...] News had leaked out, so when at last there was an official announcement of the Queen's death, the crowd chanted in derision "Queen Anne is dead - didn't you know?" and to this day "And Queen Anne is dead" is a standard rejoinder to somebody who bears stale news or states the obvious.
English Stack Exchange

Is there an Idiom or term for someone sharing information to a group that already knows it?

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