How UPDATE and check for duplicate key

Question

        CREATE PROCEDURE update_logo_id_in_feeds(feed_id INT)
          BEGIN
            UPDATE feeds SET logo_id =
            (
              SELECT image_id
              FROM posts
              WHERE id =
              (
                 SELECT post_id 
                 FROM feed_posts 
                 WHERE priority=
                    (
                       SELECT MAX(feed_posts.priority)
                       FROM feed_posts
                       WHERE feed_posts.feed_id = feed_id
                    )
                 AND feed_posts.feed_id = feed_id
              )
            )
            WHERE feeds.id = feed_id;
          END 

I have such procedure. It call when new post add to feed (write in pivot table feed_posts) and set logo_id. I have 100 feeds and when I add one post to 100 feeds - all 100 feeds have same logo_id. How check when UPDATE if other feeds with such logo_id exists and don't update?