Twice! Separate lists gives the list manager more control over the data. One major advantage is if someone unsubscribes from a specific list, they will still be subscribed on all other lists in MailChimp. Our application does allow you to merge CCB groups to one single MC list but group those CCB  groups as separate groups under that specific MC list. This way you can have one list and multiple group segments. This will allow you to not have duplicate counts but it can mean that if someone unsubscribes from that specific group, they will be unsubscribed from all groups under that list.