I'm not sure the legalities, definition, etc., of crossposting but I do know it happened to me one time. I posted something on a board and later went to another board only to find my post already there. It wasn't a big deal or secret or anything but it did kinda bother me. I probably would have posted it there myself anyway but I felt like they were my words/thoughts to share. Since then I've been very aware of it. To me it's like when a friend asks you not to tell another friend something. You may not agree with their decision but you should honor their wishes. They usually have a reason.