How is Google finding multiple addresses for my pages?

I have permalinks set to %postname% and my pages are example.com/about or example.com/contact, etc. In Google Webmaster Tools Crawl Errors of disallowed pages in robots.txt, it lists many pages like this:
example.com/?page_id=22
When I go to those URL’s, it’s actually my about us page, contact us page, or even homepage. The above link redirects to example.com/about.

I am worried that this will be seen as duplicate content. I don’t understand where it is getting that address from. This is my 1st WP site. It won’t index my homepage and I’m wondering if this is the reason.