I start at the beginning and read all the posts before commenting. That's unless I've been following the thread from the beginning, in which case I just read the new posts.
Personally, I don't like to post something if it has already been stated. However, it can be useful if there are several opinions on a subject for each individual to post their view even if it is the same as previous posts. That way you get a sense of whether there is a common view among PPRuNers.
One reason why sometimes people post the same view is that it takes time to compose, read, correct and submit your post. During that time someone else may submit a post that says the same thing as the post you're about to make.