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MailPoet's forms don't show a specific message (ie "You're already subscribed!") to a visitor who subscribes twice for privacy and security reasons.

The visitor will see the same message every time he subscribes regardless if he's already on the list and confirmed. 

Moreover, the signup confirmation email is sent again because that's what the visitor expects (ie, "please check your inbox to confirm...") although it serves no purpose unless he was previously unsubscribed.