Thats a good point by Keygrip about multiples.
I sometimes get the odd e-mail asking if it is ok to post in here and somewhere else and sometimes, I have done this quite a lot too, I move a copy of a post thinking it will get more coverage and a better response in two forums. When I do this I always tell the other forums moderator so they don't move it elsewhere or delete it or something like that.
I will only move/copy the thread if I think it will do well in two forums. The downside of doing this though is copying the post to the other forum then seeing it already there....
It sometimes can't be helped when someone new posts multiple in other forums. They should know better if they have read through the conditions when they sign up. More-often than not, they don't, and when you point the error out to them they always apologise and things are ok then. Its when regular posters do it that gets on our nerves.
I would suggest to anyone who is not sure and anyone who wants to post in more than one forum, to post in one forum then ask the mod of the other forum(s) if they can post in there too. Doing it this way shouldn't really be a problem, a bit of courtesy always goes down well.