Is it that you want the customers to receive another double opt-in confirmation email? For it to arrive in the first place, they have to click a link in an email after they sign up to confirm that they want to receive email and SMS marketing from you. The confirmation email is sent automatically when someone enters their email address and subscribes in your online store.
After this point, unless your customer re-subscribes with a different email address, there is no current way to resend this same email. However, could you please let me know why you're looking to do this? That way we might be able to find a better way around it.
Lana | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog