Why you see duplicate at the top of post?

That’s because one of them is the Featured image of post, and other is the image that you added to the content.

To avoid the duplicate images:

You always add the Featured image for the posts. But don’t add again the Featured image to the content of post. As you see here, we don’t add the Featured image (the Car) to the post content

Or you go to your dashboard, navigate to Appearance –> Customize –> Post settings –> Check the box Hide featured image…”. The Featured image will be hide on the Homepage or Individual post, so you will definitely don’t see a duplicate images anymore.