It is a common SEO mistake to have multiple URLs that all access the exact same page on your site.

An example of this is:

When Google crawls different URLs it considers each URL to be a different page. In this example it will assume that there are 5 different versions of this page that have the exact same content. Google may be able to determine that these are all the same page, but you would have to put your faith in Google to figure this out. It is better to make it as obvious as possible for search engines that there is only one homepage at and remove any doubt.

The other issue is the dilution of “link juice”. Assume that 5 sites link to your homepage but each site links to a different URL. Each different URL will get credit for 1 link. However if you only have 1 URL for your homepage, you will increase the chances that everyone will link to the main URL. You can redirect the extraneous URLs after the fact but there is a leakage of link juice every time a page is redirected. It is estimated that 90%-99% of the link juice is passed on by a 301 redirect.

Creative commons photo courtesy of Charles Sipe.