It seems like sometimes the message goes through, but appears not to-tricky for all of us
I have gotten a lot of PMs about double/triple posts as well. If you submit sometimes it says there are issues and looks like it has not gone through-trick!
It has gone through, but does not give you the "Your message has been submitted" (or whatever it says) message.
When I would use my back button and refresh the page, I would see my message but like stated above the server does not bump it up or acknowledge it was made, but it does show it.
So if you get an error message while submitting a post,
don't re-submit it without checking 1st, this will help decrease the number of duplicate posts. (but if you do have duplicate posts I am trying to go back and clean up ones that are done more than once, so don't feel bad about it)
If you want to see the last posts and *know* you have added on but don't see it, rather than clicking on the thread title, look to the right of the title and there is a little icon that looks like a piece of paper. That icon will take you to the end of the thread, and show the "disapearring" posts.
It will all be worked out intime, but knowing that even with an error message it DOES go through helps to decrease the multiple posts.
Hope that helps