Just looking at this from the point of view of a visitor, if I were already signed up to your newsletter, I wouldn't bother ticking a box saying "please also add me to the enews list", as I'm already on it.
Amnesty International have a good system (I think) on their petition pages. There's a question along the lines of "Would you like to receive email updates?" and then Yes/No radio buttons. There is also an explanation that checking "No" will unsubscribe you from their email list. So it's quite clear what action you need to take, and what the results will be.
So maybe saying "Not already a subscriber?" or something similar before the "please also add me to the enews list" would make it clear it's only aimed at new sign-ups and existing recipients can continue to unsubscribe, if they wish, via your usual mechanism.