This is what was happening on my forum:
1. If I replied to a thread without quoting anyone, it would be fine.
2. If I quoted someone, typed in my reply, then hit the Post Reply button, the Post Reply Button would be greyed out for about a few seconds, then it would turn back on again. It would look like my reply was not submitted. But if I refreshed the page, I would see my new reply.
This is causing many duplicate posts on my site. Many members are dodging the problem now by hitting F5 everytime they reply to a quote. But some people thought their replies were not submitted and hit the Post Reply button again and again -> duplicate posts.