Post image appears full size on blog list page

On the blog listing page of a site I manage, if a post has an image that’s positioned at/near the top of the post, it shows up in the blog listing page as full size. When I enable thumbnails (media settings), what shows is the thumbnail AND the full size image. I can get the full size image to not appear in the summary listing page if I position the image farther down the page, but surely I should be able to put an image at the top of the article and be able to suppress it from showing up in the summary listing?

I also tried the Auto Post Thumbnail plugin but as far as I can see it’s not doing anything at all.

I can post a link if necessary, but would prefer not to if a solution is known without having to do that (it’s not my site).

Any help would be most appreciated! Many thanks.