Thumbnails produce duplicate images

Hi! I’m using the Quarter theme and I’ve just had a problem described in a previous topic. After creating thumbnails every single post of mine got a duplicate picture at the head of the post.

I am now manually undoing those thumbnails. How do I avoid this issue from occuring? I do need the thumbnails without duplicates.

http://taleofthegreat.com – website with issue. Deleted duplicates on first pages now.

https://wordpress.org/plugins/auto-post-thumbnail/