@brandon I’ve submitted this issue to MailPoet support as well to see what they say. But doing a little bit of debugging myself (my skills are limited), I’d say that the error occurs in the eNews Extended plugin function.
I think your plugin is calling the widget( $args, $instance ) function once per each widget rendered on the page. If you increase the number of widgets, you get the corresponding number of confirmation emails.
The confirmation message is always displayed in the widget that is rendered first on the page, regardless of where you actually submit subscriber information.