Sadly there's no way for forum users to do this.
When I'm checking posts and spot a message posted twice in error, I usually delete one.
If you make a mistake you think is serious or important to correct, you can email me and I'll change it for you.
You'd need to provide me with the following info:
Forum
Subject
Old post (in full)
Replacement post (in full)
Moderator1
pcvforum@templenet.co.uk