Yeah, there's lots of cool little features like that.
To answer your question "what do we call two successive posts?"
Answer: Two successive posts!
Or actually, to me anyway, it's just continuing the conversation.
For example, if you had just continued to edit your post #8 and not added the next two, I never would have seen them because I have my preferences (Under MySpace) set to send me email notification of all replies. And edit does not show up as a new reply so I would have missed your continued dialogue.
It is my understanding (and other people in other places might view it differentl) that a Double Post is when the same question is posted successively or in two different threads as an attempt to get an answer quicker, which becomes very frustrating for the readers cuz then we don't know which one to respond to when in two different threads, like I mentioned above.
Also a double post (like "anyone? please help? did you see my question?" posted right below the post they just made 11 minutes earlier ) or "bump" can be annoying when it's just due to the poster's impatience, however to me, there are definitely times when a "bump" to the top is beneficial, such as bringing attention to a thread that is relevant to someone that has just entered the forum.
But as you said, there is the nifty little "related topic" link function that you mention that kind of serves the same purpose as bumping a topic to the top of the forum.
I like to bump topics to the top when they are full of good info and there seems to be a lot of people coming in and asking about that part of the game again. I'll go find a thread that I know about and just enter a post that says "bump" and then it brings it to the top cuz that's the most recent post.