This issue has come up before and it seems like there are 2 *very* distinct trains of thought 1 - it is rude to post without reading all the way through (for the reasons you gave ie people don't realize things are being repeated/the OP has already said the won't work/OP has clarified something) 2 - posters who say that "you can't dictate the way other people post." I do not believe a consensus has ever really been reached.
I can think both those things.
In general, you shouldn't ignore other posters. Especially the person who originated the thread, but also those who have already weighed in.
However, I don't think I've ever seen anyone be admonished or disciplined here for failing to do that.* I'm not sure whether it's that the mods haven't been alerted to a thread where that has happened or not. And the threads do get terribly unwieldy sometimes. I have been known to skim, but if I'm actually going to reply I try to read all previous posts.
It's important to give the mods a heads-up if you want a ruling on something. They don't read every thread.
*(The case Rainha is alluding to was, if I'm not mistaken, a situation where a poster committed a particular violation of a forum content rule and was publicly admonished and banned. Another poster came along a few hours later and repeated the violation. And was banned. If she'd been paying attention, she would have seen that a particular subject was off-limits on this board. A very good
advertisement for reading previous posts. And the rules.