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Hello,
Thanks for reaching out to us.
This will require additional custom code. You can find an example at https://github.com/ibericode/mailchimp-for-wordpress/blob/master/sample-code-snippets/forms/custom-validation.php.
You will also need to add another email address field using HTML code to the form and then use the custom code (that can be placed inside the functions.php) file of your active theme to validate and compare data between the main email address field and the 2nd field.