Duplicate or double images.
Featured Image Repeated in Body
Posts are showing a featured image and a body image.
By default the plugin uses the "featured image" (you would add these using a setting in the post). If the same image is also repeated in the body, and the email displays body content, you will see duplicate images. Remove the duplicate body image or use the Premium option to “use first image in body” to bypass the featured image.
Can't delete the body image because your web page doesn't display a featured image?
Use the Premium plugin option "feature first image" to send the first body image to the feed instead of a featured image.
Media Tag Repeated with Featured Image
If both a featured image and media tag are sent to the feed it could result in double images displaying in some services, like Mailchimp. Checkbox the setting "use only media tag images”.
Another plugin or the theme could also include the image in the feed, even if that’s not their primary purpose. Common ones are "RSS Link plugin" and “Feedzy." Go through your other plugins and theme settings. If necessary disable other plugins one at a time, clear your cache, and test the feed to find the culprit.
Continuing the troubleshooting for conflicting plugins, check your theme's functions.php file for thumbnail RSS code. It can be a less obvious cause of duplicate images.