For us humans, these URL below are all the same webpage. It is a link to our website's main page or home page.

http://yourdomain.com
http://www.yourdomain.com
http://yourdomain.com/index.html
http://www.yourdomain.com/index.html

For search engines such as google, all of these are different.

A lot of people are suffering from duplicate penalty because of this. People assume that the search engines look at it the same way as we humans do.

The best way to avoid this is to stick to one format. Most of the time it would be http://www.yourdomain.com. If you have a *HOME* button on all of your pages, check if it goes to the same homepage URL everytime. If you have other websites that link to your website, check the URL also. If you are asking people to link to you, make sure they all link using *ONE* format only.

Apart from the duplicate penalty, the page rank is also divided among those different URLs. Instead of having a PR4, you might have a PR3 for www.yourdomain.com and a separate PR3 for www.yourdomain.com/index.html

A quick fix would be to use a 301 redirect.

You need to do a permanent redirect from http://www.yourdomain.com/index.html to http://www.yourdomain.com.

On an Apache server, this can be done via the .htaccess file. I won't get into much details but the entry should be something like this:

redirect permanent /index.html http://www.yourdomain.com/

Note that this also applies to any of your subdirectories also.

Example:

http://www.yourdomain.com/directory/

and

http://www.yourdomain.com/directory/index.html

So make sure you be consistent about using one format only. Do advise your web development team about this so it would be consistent as you add more pages to your website.