My feature images are showing up twice on the preview page

I think you given like

echo the_post_thumbnail($post_id);

No need of giving echo if we are using the_post_thumbnail().It by default echoing the result.So here, it will display the image twice.
If we use get_the_post_thumbnail(), then echo is needed.

So use either

echo get_the_post_thumbnail();

or

the_post_thumbnail();

Try this..