The duplication may stem from whether one chooses "reply," "add reply," or "quick reply." I for one am unclear on which one of these does what. Also, this post will be a test (to see if we/I receive a duplicate again), as I've merely "reply" 'd to OD last post, without adding or changing the title line.
I have a hunch (or a conspiracy theory) that when I not only reply, but also change the title in the title box, this action may generate two posts to be sent...one which replies to the former title (if any), and one that creates a new title w/ corresponding duplicate post.
Then there's the other side of me that says it's my yahoo account's fault, like roz.