This has been identified as an issue with the forums.
What happens is when you click "New post" (or any other link that will let you make a message) your future post is assigned an ID.
The problem is how this ID is generated. At the moment the forums will look at the ID of the last post and return the ID +1. Trouble is if someone else starts making a new post and gets the same ID (because you haven't pressed "submit" yet) then when you press submit the ID already exists.
Until post IDs are assigned at time of post submission ONLY, the only way to get around this is:
If you get a "Duplicate post ID" message, when you return to your message copy the contents of the post.
Then go back and start your reply from scratch and just paste the contents back in. (this is nice and fast and you shouldn't get a seccond warning message).
For someone like me who has to look stuff up during a post (and my posts are usually fairly long) this bug is endlessly annoying, so I feel your pain.