When an already subscribed user comes back to purchase on your shop, no action will take place on MailChimp list and you will get an error within the order which notifies that user has been previously subscribed.
The control will be applied in the email field.
The error doesn’t happen if the subscription list has changed compared with the previous purchase.
However, in order to avoid the above mentioned MailChimp error, youcan enable the “Update existing” option.
This way the subscribed user’s profile will also be updated automatically in case he inserted a different name/last name, compared to the last purchase.