Nutch would benefit from having a host normalizer. A host normalizer maps a given host to the desired host. A basic example is to map www.apache.org to apache.org. The Apache website is one of many on the internet that has a duplicate website on the same domain just because it allows both www and non-www to return HTTP 200 and proper content.
It is also able to handle wildcards such as *.example.org to example.org if there are multiple sub domains that actually point to the same website.
Large internet crawls tend to get polluted very quickly due to these problems. It also leads to skewed scores in the webgraph as different websites link to different versions of the same duplicate website.