Most the time it's because you click the "save" button twice, say for instance by accident, or you click it once, think you didn't click it, and then click it again.
It can also happen when a browser sends a faulty packet, but that is extremely rare: It's mostly due to user error.

A way to avoid double posting is to always assume a comment is posted the moment you press the save button, regardless of whether the page successfully reloads or not (which is usually the case, sending your comment is far faster, often only taking a split second, and takes far less bandwidth than reloading the page)

If you have a feeling you've posted once already, navigate back to the thread (saving your comment, of course) and checking if the comment has been posted already.

BTW it is not possible to double post when you are editing a post, so spam the save button as much as you want.