Believe it or not, I see many websites that have multiple homepage URL’s. You might think, how is that possible? The reason how is because every website has an index page at the root. It depends on how each server and CMS handles it. The problem is that Google considers each of the following URLs as separate URLs, even though they all resolve to the Homepage.
The www and the non-www are duplicate. As well as the /index and /default subdirectories. This can cause duplicate content issues, and dilute the strength of your homepage. Which by the way is the most powerful webpage there is on your site.
Diluting the strength of your homepage can happen if your inbound links get split up between several homepage versions. That’s in addition to the issue of duplicate content.
One way to resolve this issue is to choose a Canonical Homepage URL, and 301 redirect all other versions to the canonical version. So, if you choose www.example.com as your canonical URL, then all other versions of your homepage should 301 redirect to www.example.com.