Problem 1:
Non-www and www Versions of Site URLs
If your site has a non-www and www version of all your URLs, you’re going to be splitting link value for the same content between two URLs. Rather than capturing 100% of the link value on the page you want to rank in the search results, you could be doing a 50/50, 60/40, or some other kind of split between the two URLs.
Problem 2:
Duplicate Home Page URLs
This is a close cousin to Problem 1, but is especially bad because it’s your home page.
It is possible to have a lot of different URLs that all go to your home page content, especially if you haven’t fixed your www and non-www duplication, which can result in a lot of unnecessary duplication.
For example:
• http://www.mysite.com
• http://mysite.com
• http://www.mysite.com/index.html
• http://mysite.com/index.html
These URLs will all lead to the same content. If you have links going to all four of them you will see a four-way split in link value. There are a lot of sites out there that have fixed their non-www and www problems, but I can still find multiple versions of their home page.
I’ve even come across sites with different extensions (.php, .html, .htm, etc.) that bring up the same page as well. This leads to even more duplication and potentially wasted link value.