This issue is because your theme displays post’s featured images in the top of the post, and also, the post contains the same image, in the post content, once again. To fix this, you have two options: you can edit your theme’s settings, and make it not display featured images for single posts (each theme has it’s own configuration, so it is hard to tell how to do this in your case). The second option to fix this is to check the ‘Strip Featured Image From Generated Post Content’ checkbox, from the plugin’s ‘Main Settings’. This will try to remove the featured image from the post body.
Also, if the above feature is not available in the plugin you are using, please look for the ‘Strip Images From Generated Post Content’ checkbox, in the plugin’s ‘Main Settings’.