It is possible to have invisible text on web pages, this hidden text is invisible to human viewers, and is meant purely for the benefit of search engines to increase keyword density. The most common method of invisibly embedding keywords and phrases in the HTML page is to use a font colour identical or nearly matching the background colour of the web page, making the text invisible to viewers, with the intention that the text will be parsed and indexed only by the web crawlers. Search engines are on the lookout for hidden text and can penalise any web page that contains it.
Doorway pages are pages specifically designed to be attractive to search engines. When a user clicks on a link in the search engine results page, the user is automatically redirected to another page, (or another website) and never actually gets to see the same content as the search engine spider.
Over Optimised Pages
All major search engines are aware of attempts by webmasters to achieve a higher ranking in the search results page, so avoid attempting to create perfectly produced web pages, for example, when the web page keywords are repeated to an unnatural extent (commonly known as "keyword stuffing"). Attempt to create pages that read naturally to humans, do not produce web pages specifically for search engines.
The search engines already have billions of web pages in their index and when they index pages that have substantially identical content to an existing page, they will remove the pages (and possibly the website) containing the duplicated content from their database.