If two URLs in the same site are referring to the same page without any redirect header, like example.com/index.php and example.com/index/my-page/ displaying exactly the same content, both links available in the site, how does it affect SEO?
It is not good for your site to have duplicate content on your pages. Ultimately it will effect your whole site. It may also cause un-indexeing of your pages from Google's database.
david is right, it will affect your site badly...still google will crawled it but it will not get indexed...google will ignore...
Google does not want to show duplicate content on its search result pages so what will happen most likely is that it will pick one of the two pages as the authority page and display that page. If there is a lot of duplication (as in many urls pointing to the same content) then the search engines might drop all the pages form the index. So, if you can, avoid duplicate content.