Idiom meaning to talk about something everyone already knows

Thanks, Cpt. Obvious!

Loosely related to the expounder Mr. Exposition, a TV trope that provides infodumps or expounds the plot. An agitator or lamentor.

Additionally, AYKB (As you know, Bob), discussed here (Info dumps, Soap-boxing, Lecturing).

Sometimes described as the Turkey City Lexicon, or a FAQ Literator.

Loosely related concepts are: perverbs or Wellerisms (making fun of established clichés and proverbs by showing that they are wrong in certain situations, often when taken literally) and Tom Swifties (a speaker attribution that puns on the quoted statement).

There's the ever-popular act of beating a dead horse...

1 : to keep talking about a subject that has already been discussed or decided
_{from m-w.com}

I would say that they are harping on about something (for whatever motive).

If you say that someone harps on a subject, or harps on about it, you mean that they keep on talking about it in a way that other people find annoying.

Collins

Trump began promoting Moore by harping on how bad it would be to have a Democratic senator in Alabama.

Axios 12th December

The Ngram shows that there is equivalence between AmE usage and BrE.

My first thought on seeing the title was preaching to the choir.

However in the context(s) you went on to describe, @Hellion's answer is better. I've tried to upvote but apparently don't have enough reputation for it to affect the displayed score.

Not meaning to state the obvious, but the idiom I'd use for this behaviour is "stating the obvious."