Hope you are doing well!
I came across your post and will try my best to help you out with this bug.
Could you please share your website URL along with other details such as you are using some apps for both the Newsletters or you are adding the customer's email address into Shopify customers because in the latter case, if the email address is added once to the Shopify customers list then it won't be duplicated but if you are using different apps then it should definitely go into the separate list.
Hope it helps! Looking forward to your response.
Thanks for providing details. Well, unfortunately this is not possible in Shopify to add an extra tag to existing customer email address.
I would recommend you to add MailChimp as one of the Newsletter subscription so that customer email address can go to two separate lists.
Hope it helps!