Duplicate attribute srcset with thumbnails

When using WP Retina 2x HTML srcset method with the_post_thumbnail(‘thumbnail’), I get the validation error “Duplicate attribute srcset”.

When using the_post_thumbnail(), I don’t get the validation error “Duplicate attribute srcset”, but I need to specify the ‘thumbnail’ size.

(The duplicate attribute can not be seen by viewing source in the browser. It can only be seen by validating at http://validator.w3.org/.)

Validate this page to see the error: http://craftlibrarian.com/sleep-like-a-tiger/

The same duplicate srcset is in WooCommerce here: http://demo.swampsidestudio.com/product-category/products-for-therapists-and-counselors/

Any ideas how to prevent the duplicate attribute srcset?

https://ww.wp.xz.cn/plugins/wp-retina-2x/