If i had a copy of my website in a subdomain, will that hurt my SEO?

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Yes and no...but mostly yes.

Google is rather evolved say in the case of a wordpress blog. It generally knows that you may have some potential on-site duplicate issues...and won't take away your birthday for it.

That being said you definitely want to ensure you focus on having that text in one spot...esp if you're consciously publishing it in both spots.

If you are using one location as kind of a draft or private copy for a group of your coaching students, or whatever....consider making sure you exclude that location in your robots.txt and also if you have it linked ANYWHERE on your site apply the rel="nofollow" tag to the <a href="http://yourotherlocation.to-article.com" rel="nofollow">yada yada</a>

Hope that makes sense.