There are a couple of situations where you might see this message:
1. If you happen to inadvertently hit the Post button more than once
2. If you're trying to copy/paste from a Word document without using the Word Editor in the toolbar.
If neither of these situations apply, please describe what's going on before yoou get the message.
I was trying to reply to a thread and I kept getting that message.
I saw the first time someone must have posted at the exact time as I did, so I understood why I got it. But, I kept getting it later when I tried again. I would wait 2 or 3 minutes and try again and still get it. I finally just gave up.
It isn't the first time it has happened and I have no clue as to why.
At this point, I can only say it must have been some kind of glitch with the server. However, if it keeps happening, or others also post about the issue, I'll file a ticket to investigate it further.