No error message being received on duplicate signup

[aliasmac]

When I call addToMailchimp with an email address that's already been added to my list the error message "xyz has already been subscribed to the the list." is no longer thrown, instead I get the success message {result: 'success', msg: 'Thank you for subscribing!'}.

Has anybody else encountered this?

I also tested with the Mailchimp Marketing api and when I hit the equivalent call with mailchimp.lists.addListMember I get the expected error message on a duplicate signup which rules out anything related to my Mailchimp account settings.

It was working a few days ago and the other error messages filter through just fine so it's a bit unclear why this is now happening :/