It's a reference to the fact that the default document of a site (or of a directory within a site) can be reached both at the address of the site (or directory) or at the full URL of the document itself. So it's not really different versions of the page -- it's just one page, but the server may display it at different URLs. A search engine that finds the page at more than one URL might conclude that that it's the other way around: that you've got more than one document containing identical content.
How would you know if your server does this? Just check to see if the page can be reached at both URLs, without a redirect taking place. When you request site.com/index.html (or/default.aspx, or whatever your server is set up to call a default document), does the page display with that exact URL staying in the location bar, or did the server get the request for site.com/index.html and redirect you to site.com?
You should also check to see if the search engine gets that it's just one page. When you search for cache:www.site.com and cache:www.site.com/index.html, do you get results for both? At the top of that result, do you see the URL you asked for at the end of "This is Google's cache of..." or does it tell you that you're looking at the cache of site.com when you request the cache of site.com/index.html?
Edited by qwerty, 07 April 2013 - 09:06 PM.