We've got a customer asking if it is possible to prevent subscribing to the same newsletter twice. The subscription part all works fine and users can subscribe themselves to a newsletter. However it is possible to subscribe with the same email address twice. The second time it is still possible to subscribe, customers also getting the confirmation e-mails for the second time, eventhough i've already subscribed to the newsletter. Is it possible to show the visitors a message telling them that they've already been subscribed to the newsletter and that they can't be subscribed again?
