poush makes a good point if Flarum eventually goes down the route of merging posts at a DB level. It annoys me when I post something, then think of some important bit of detail I missed and I know people are reading my post (busy forums etc) and they're not notified that it has been edited. I would like the post time to change or at least for the topic to display that it has new posts - perhaps that might be a little confusing if you're taken to the same post you read but with minor differences, so perhaps an "updated" time might be handy?
Since the poster who is posting again in quick succession has no choice with this sort of feature over whether it's merged or not you could be adding something hugely important you forgot to mention a minute earlier and it just gets lost in with your previous post. If Flarum eventually merged posts that way that is.
The visual merging and not actually merging them behind the scenes solution works just as well for all this though, perhaps better the more I think about it, but thought this was worth mentioning.