I observe similar from time to time. Not sure if it's a jumpy finger on the mouse, or what.
I have had that happen, but it was almost always when I got logged-out, while posting. After logging-in, I'd check and not see my post, so I'd hit Post Reply, only to THEN see both - simple matter to Delete and add "Post Deleted - Duplicate." Twice, I had used the Back button, got interrupted, and when I got back, saw my edit screen and just hit Post Reply. Oops, double-post, so I handled it like above.
Now, there are enough examples, like the one that you cite, that makes me wonder if it's maybe a browser, a reply via e-mail, or something else - or that twitch in the mouse finger? Maybe even a Jive-deal, when the phases of the Moon are just right?