Great question — answering as the UphillTech team:
I'd need to see your site to be sure, but there's a very good chance it's your theme (not WordPress itself) that's setting the extra description field. You can remove that from your header.php file and you should be set. WordPress itself doesn't have SEO features like that built-in.
Full answer by EAMann: https://wordpress.stackexchange.com/a/758 (CC BY-SA 4.0).
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