Upload & Sell: On
Hell, I've seen ads where guys have posted up several items in a row. Give it a try. Worst case, it won't work
I didn't get the impression that OP wants to double post, rather, that he saw a double post and wondered if that was in accordance to B/S rules.
I don't think it is explicitly prohibited, but there are some people who list the same item in multiple FS ads. If not against the rules, it's probably at least poor form