SEO experts sometimes demand that default document, e.g. www.site.com/index.html redirects with 301 to www.site.com, so:
- Is that really necessary?
- Does Google really penalize that?
They were right to do that although a canonical URL would have also sufficed. Technically speaking those are two different URLs and thus considered two different pages. So the 301 redirect, or canonical URL, tells Google that both are the same page and to handle it accordingly.
While all the answers on this page are correct this is such a common issue that I think Google is smart enough to work things out here =)