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you mean on this site or just newbb in general?
i've not used newbb very much and was a while ago now, but i don't think members can, but moderators are able to..
most members on this site if they make a post in mistake or double post, they usually just edit the post in question and leave a message or something..
ie..
*EDIT* sorry this post was made by mistake or sorry double posted..*
i don't think anybody will pull you up abt it if that's just the case :)
the new newbb i think has a delete feature.. again, there might be a setting in newbb to allow deleting of ur own post but i can't remember..