Another cool idea from Hulk. On my current forum I have this feature - if someone sends a post and then sends another one in 24 hours, if there’s no replies between, this posts will merge like this:
First post.
– added after 30 minutes –
Another post.
I require this in Discourse! 
I think the idea is to teach users to not do it. How Discourse solves it is by warning you about posting more than twice in a row. Though I could be off on the specifics.
Teaching method is lame. 
Because noone wanna learn, people want big fat button on a center of screen that do everything for them. And I think giving them this opportunity is the best way of doing things. IMO.
I guess we’re all doomed then 
If this is something you want to do on your forum, then write a plugin for it.
I don’t want this type of functionality on my forums. New posts will just be new posts - even if nobody replies 
This is already covered many times, search for “merge sequential” or “merge multiple”. Not opposed but it is harder than you think to implement.
Discourse warns about three sequential posts by default.