How can i find posts with duplicate featured images?

The featured image’s ID is saved in a post meta field by the name of _thumbnail_id, so with a little bit of joining, you can use MySQL to search them for you:

SELECT p1.ID as ID1, p2.ID AS ID2 FROM
wp_posts p1 LEFT JOIN wp_postmeta pm1 ON (p1.ID = pm1.post_id AND pm1.meta_key = '_thumbnail_id'),
wp_posts p2 LEFT JOIN wp_postmeta pm2 ON (p2.ID = pm2.post_id AND pm2.meta_key = '_thumbnail_id')
WHERE
p1.post_type = p2.post_type AND
p1.ID < p2.ID AND
pm1.meta_value = pm2.meta_value AND
pm1.meta_value IS NOT NULL;

This looks for shared featured images within the same post_type, you can just remove that condition if you want to look across all post types. p1.ID < p2.ID is in there so we don’t get repeated reports (e.g. “1, 2” and “2, 1”), and it makes sure that we don’t compare a post with itself.