Add "You have already subscribed!" post-message if someone tries to re-subscribe

[Milian]

Hi,  
In the Prestige theme language settings, it seems there's only a post-submit message if someone has 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?

[StephensWorld]

This thread includes steps for how to achieve this: 

https://community.shopify.com/c/shopify-design/newsletter-language-for-already-subscribed-users/m-p/... 

[Milian]

Thank you for your reply, but this code is not working for us.