WordPress SEO by Yoast allows you to add a "noindex, follow" to these pages. that will prevent the duplicate issues. You can also remove the media pages from your sitemap.
You can even drill down to each individual image and determine whether you want that image to be indexed or not.
Another option is to remove the link from the image in the post. I know you are dealing with quite a few images but unless you are wanting to promote that image, I would remove the link.