Every time Google indexes a page, it checks to see if the page is already in its database, if Google detects duplicate content, or notices a page that has not been accessed in a long time, Google automatically marks the content as a supplemental result. More and more webmasters have expressed their displeasure at their sites being labelled as supplemental results. The truth is that Google almost always labels a website as a supplemental result because it detects duplication of content or design elements.
Duplication of content here could mean many things, the most popular is making multiple pages on the same website, as this improves the keyword density of the website, and hence improves its chances of coming up multiple times for the same keyword. This is one of the oldest ‘black hat’ search engine optimization techniques in existence; only thing is that Google does not allow such methods any more. The moment Google detects similar content; it automatically labels all the similar content as a supplemental result. This means that not only the duplicate pages, but also the original page or website becomes a supplemental result!
Another method that results in Google labelling a website or page as a supplemental result is by giving all the pages on a site the same heading, or assigning similar attributes. This causes Google to detect (sometimes incorrectly) that the same page name has been used with different content, another search engine optimization technique that is no longer of any use. In fact this also proves that the term duplication applies not just to content, but also similar structure.
If you are wondering why blog sites, and forums are not reported as supplemental results, simple Google crawlers do actually have AI (artificial intelligence) they detect and they know which sites are blogs and forums, and which ones are simply abusing the power of content, by creating duplicate content. So in essence its becoming increasingly difficult for webmasters to fool Google. Another reason for Google marking sites or pages as supplemental results is when the site receives very low traffic, Google naturally assumes that a page that has not been visited in months, can hardly be useful and automatically catalogues it as a supplemental result, however these types of supplemental results are rare, and the simplest way to get out of supplemental results is to increase the web traffic by using search engine optimization.
How to get out of Google supplemental results
If your site has been labelled as a supplemental result, the first thing you need to do is to remove any duplicate content, or duplicate web structure from your site(s). Google is a fair search engine and it does start indexing sites once it detects that a site is no longer duplicating content. In addition increasing the traffic to the site and improving the overall structure of the site also helps speed up the processing of Google, but the single most important way to get out of Google supplemental results is to delete duplicate content.
Many webmasters have tried smarter techniques like redirecting pages to a new address, or deleting old content, and creating new pages with the same content but different file names, all these techniques are doomed to failure as Google will eventually detect the duplicate content, and might no be as lenient this time around. So once you are in supplement it is best to do the honourable thing and delete all duplicate content. Also using white hat search engine optimization techniques, and improving the overall traffic to the site, should speed up the recovery process of the site.