One consideration many humans working their SEO are not aware of is what duplicate substance can do to their search results. The same subject matter on many different URLs makes Google think you are a spammer and they will not hesitate to soundly punish you for it.

Pandia has a great article on that topic.

The search engines hate finding two pages with the same or similar subject matter. Unfortunately contemporary blog software and composition management systems have a tendency of generating many different URLs for the same page. So what is a webmaster to do?