Duplicate Content and Google

[mandy]

The most important things to know about duplicate content are:

    * Google wants to serve up unique results and does a great job of picking a version of your content to show if your sites includes duplication. If you don't want to worry about sorting through duplication on your site, you can let us worry about it instead.
    * Duplicate content doesn't cause your site to be penalized. If duplicate pages are detected, one version will be returned in the search results to ensure variety for searchers.
    * Duplicate content doesn't cause your site to be placed in the supplemental index. Duplication may indirectly influence this however, if links to your pages are split among the various versions, causing lower per-page PageRank.

full article: http://googlewebmastercentral.blogspot.c...t-smx.html

[mandy]

More on duplicate content:

How can URL parameters, like session IDs or tracking IDs, cause duplicate content?
When user and/or tracking information is stored through URL parameters, duplicate content can arise because the same page is accessible through numerous URLs. It's what Adam Lasnik referred to in "Deftly Dealing with Duplicate Content" as "store items shown (and -- worse yet -- linked) via multiple distinct URLs." In the example below, URL parameters create three URLs which access the same product page.

Why should you care?
When search engines crawl identical content through varied URLs, there may be several negative effects:

1. Having multiple URLs can dilute link popularity. For example, in the diagram above, rather than 50 links to your intended display URL, the 50 links may be divided three ways among the three distinct URLs.

2. Search results may display user-unfriendly URLs (long URLs with tracking IDs, session IDs)
* Decreases chances of user selecting the listing
* Offsets branding efforts

full article: http://googlewebmastercentral.blogspot.c...y-url.html