I see nothing wrong with continuing the tradition of doing something like “Pssst! Check out post #123” which many of us do. It’s accurate, it’s not quite shaming, and more than a few of us respond with an appropriate amount of chagrin.
But it does bother me. Because I -will- read an entire multi-hundred post thread to make sure someone hasn’t already made the point, though I’m sure being human I’ve screwed up at least once.
Me too. If I thread I would like to post in hs grown too long to read before I even saw it, I don’t post in it.
I do this too, though I have overlooked a post a couple times, double-posted, and gotten a “psst:see post #123”. Another thing I often do in very large threads is do an in-thread search for keywords. That’s very effective, except of course for the rare instances someone has expressed a similar thought using completely different terms.
Of course, it REALLY pisses me off when somebody posts something that I had posted upthread. But it’s more than a minor annoyance when somebody makes the same point as a previous poster.
How about something like “Yes, already noted/linked in post # xxx.” As @pulykamell stated, it seems to be the same folks who continually do this. Perhaps continual chastisement will make them change their ways.
Can’t hurt, anyway.
This. At least try to see if your point was already made. Doesn’t mean you can’t post if it has, but it’s even better if you can quote an earlier post and go on from there. e.g.
As @somebody said in post [quoted snip here], it might be a [whatever], but I also think we ought to consider these adjacent ideas of blah, bleh, and blurf …
If they don’t read the other posts, they aren’t going to read the chastisements either.
I do this too, especially if it’s going to be a thread that involves naming movies/people/etc. where it’s pretty easy to search for proper nouns because those will definitionally be the same 
That said, like… I’ll admit I would be a little irritated if I was MrDibble and saw someone else show up to collect their prize for being the 20th person to ask “what if docahedra were designed to hold candles while you knitted a pouch for your tapered-stick measuring device,” but a 971-post thread on an arcane topic with the expectation that any new posts will also contribute new information seems destined to be a 1,000 post thread where the last 29 posts are just someone bumping to ask “so: anybody figured these out yet?”
If the post was a reply to their post, it’s a good bet that they’ll read it.
Add me to the list of those who will also read the whole damned thing before posting; or else not post.
I may be – probably will be – skimming some of it. I may miss something. But I’m at least going to try.
– sometimes this technique gets me so late into a thread that I don’t bother posting by the time I’ve caught up.
Ok, I was right there with you on your ideas ‘blah’ and ‘bleh’, but I’m sorry, ‘blurf’ is a bridge too far 
Huh. I was told explicity to answer the OP, any other thing was threadshitting.
Surprise, surprise.
Can we sticky this? 
I have no problem with posters using AI as a research tool but, agreed, posting verbatim without checking up on the facts is next-to-useless. AI has led me to an answer many times when checking up on my own questions, but it always requires either trying the solution (if it’s a solution that can be tried out) or by verifying with traditional sources.
That should be “desperate need to throw their two cents in,” of course. That’s what I get for posting at like 4 a.m. in the morning in between fits of sleep.
One can answer the OP with respect to the conversation that has already ensued, it one can ignore everything that has already been said. It’s the latter that annoys people.
Sorry for double post. I do understand sometimes missing something in the OP. It does happen. I’m sure I’ve done it before. But the general rules, in order of important, should be something like:
Read the actual OP. Not just the thread title. Read it, don’t skim it (I’ve been burned once or twice, I believe by this). Else you end up with threads like “I’m looking for a solution to problem A. I’ve tried X. X did not work. Does anyone have any ideas?” Reply 1: “Have you tried X?”
Read at least the first five replies. I know some threads are long and it’s hard to read the whole thread. But just check if it wasn’t one of the first things mentioned in the thread. You’re probably not the first person to reach out with a clever idea.
Read about five posts before the end of the thread. You can get a sense of where the conversation has gone, what has been mentioned, and where we are in terms of finding an answer. I have found many a poster wade into a 75+ reply thread having obvoiusly not read any of it, as if they couldn’t be arsed to do the work of properly following a conversation, and then suggest something that literally has just been suggested (or, as in 2, one of the first things suggested in the thread)
This is just bonus points territory now, if your answer has any keywords that could be searched, use the search function and search the thread for the keyword to see if anyone has already mentioned it yet.
This is just my own nugget of wisdom: if you feel the urge to start your sentence “I don’t know if it has been mentioned yet,” it has. Possibly multiple times. Maybe even in bold.
Of course, we’re all allowed some grace. To err is human. To err repeatedly on the same points is willful ignorance.
For “bonus” points:
“I can’t believe that, 100 posts in, no one has mentioned [XYZ]!”
Next reply:
“Pssst…[XYZ] was mentioned in the first five posts.”
Is that deliberate irony? It’s basically #5. We could reword it to work it explictly in.
Edited quote to update with new verbiage.
No, not trying to be ironic, just giving a more extreme example, where a poster declares “my thing hasn’t been mentioned,” when in fact, it has.
Yeah, I can’t believe I forgot my own phrasing, but I wrote it in 2021 (and am probably not the only one to express this notion) as:
The thing that’s especially infuriating about that is that’s in the context of an IMHO (or Cafe Society) thread where people are asked to mention bests or worsts or whatnot and is exactly the kind of thread where searching the thread to see if your contribution has already been mentioned is easy.
Amen on this point. You and I don’t agree on much (despite rarely having interacted on the board), but I had a comment deleted from a Reddit thread, and I messaged a moderator to make my case. They upheld the deletion, so I asked for clarification on specifically wasn’t allowed in my comment. I was then banned from that particular subreddit under the guise of “harassing the moderator.”
There’s no place I can appeal that decision, either.