Pages go supplemental when the site has same similar & less content pages.
How to avoid Google Supplemental Hell?
There are few considerations which would seek beneficial:
- Installing a 301redirect from non-www to www
- Not to use index.htm/index.html
- Link only to the main page ,i.e., http://www.domain.com
- Use "NOINDEX" meta robots tags on duplicate pages
- Check for duplicacy
- Multiple URL's should not refer to same page
- Long session id's should be removed
- Too many parameters should not be there in URL
- Same content should not be there on multiple domains
- Moreover "Meta-Description" tags should be not less than 60 characters
- CSS/JS files should be saved externally
- Place more content on home page