Ok, I'm pretty sure that this has been happening to me for a while now, but since I don't do start topics very often, I didn't exactly realize there was a problem.
Starting a topic causes my browser to have some kind of error where the parent forum doesn't reload afterwords. There have been occasions where I refreshed a minute or two later and somehow this caused a double post of the topic. At least one of these was when I used the admin power to move a topic and ended up with two move announcement topics being generated.
One of the places I browse from generates the following error (with some additional info that I don't think applies)
Network Error
tcp_error
Exception: A communication error occurred: ""
Cause: The Web Server may be down, too busy, or experiencing other problems preventing it from responding to requests. You may wish to try again at a later time.
HTTP Error Code: 503
Primary Contact: Click here for additional guidance and procedures for requesting access to blocked content.
I did some tests in the curator section to see if this repeats, and to capture the above message since I had closed the tab it had happened to me on earlier. One of the tests showed that when I generate a topic from my iPhone, I don't get the above message, I just get a blank page.