Duplicate images

Sorry just seeing this. It’s probably due to outputting the Featured Image along with an image in the body of the post, I’d remove one or the other and see if that helps.

If it’s doing it without the plugin activated, then it’s most likely theme or other plugin issue, try deactivating other plugins, or try it with the Twenty Fourteen theme, and see if it goes away. If so, narrow it down further. If not, it may be a Feedburner or cache issue, try viewing the raw feed URL and review the HTML code itself.