Newsletter Language for Already Subscribed Users (sense theme)

Hi,

In the theme language settings, it seems there’s only a post-submit message if someone has NOT subscribed to our mailing list.

But when a subscribed e-mail tries to sign up again, the page refreshes and does nothing and does not give an error message like “You have already subscribed!”

How can I add a “You have already subscribed!” post-message if someone tries to re-subscribe?

I am using the sense theme.