To make it clearer, this is what I’m talking about.
I created a test site where I created a post with a single image and used the same image as the “featured”.
Both are added to the site, with the “duplicate” having “-2” at the end. Then there are almost 2x the number of responsive images created.
For this test it isn’t a big deal, but the much larger photos I will be adding to my blog are going to start to add up quick,