You know, there's a way to get it not kick in.
Just click edit on your previous post. It'll only do it when you are the last person to reply already, so you have to ask, why are you replying again to yourself?
It can only be when you've got something else to add. So perhaps just edit your original post.
I made a guide on the sparky forum for it here:
How to avoid double post merging...
Replying twice in a row in a thread will also affect your post to likes ratio.
But the main reason we have it, is to make sure the posts are longer than a sentence long each. Which is bad when it comes to ranking in search engines.
We have a nice big long first post, then lots of people replying with just a sentence. And that makes the whole thread have a bad ranking because search engines think the page isn't interesting if people haven't posted a long post.
So just edit your original post. Depending on your main usergroup the time to edit your post is up to about 90 minutes. If you're a newish member (low post counts) you might find it's around 20 minutes.
But the feature isn't going away anytime soon so get used to it.
Again, read this guide here:
How to avoid double post merging...