Just read the forum and you will see the same question a couple of days ago.
For simple logic;
– If somone subscribes for the second time for the same newsletter it’s, why would you give that person the feedback “you forgot you already subscribed and confrimed for this newsletter”. No harm by not mentioning.
– If someone else checks if you are subscribed to a newsletter by entering your emailadress and that person get’s the message “you are already subscribed” than you have a privacy-issue. That would be a problem.
– And more difficult, if someone can subscribe to more newsletters at the same time (most of us will use that option) and get’s an error because he already subscribed to one of them but not to the others…
The big question is, why would you want it when it changes nothing in a positive way.
(and also the reason why unsubscribing should never be possible by entering only the emailadres in een box on the website, but be checked before)