I ran across something interesting last night.  A Google search for "site:developers.sun.com/ajax/" returned two entries for the homepage.  One was a standard entry that I expected.  However, the second displayed the URL with a campaign ID (CID) appended (developers.sun.com/ajax/index.jsp?cid=216424).  Apparently, the Googlebot followed a link placed as part of a campaign that was being tracked through our web analytics software.

As we all know, Google frowns upon duplicate content and ranks pages lower for this.  I'm not sure if this is considered duplicate content, but seeing duplicate entries makes me think so.  To remedy this, I suggest coding CID URLs so that search engines will ignore them.  To do this, insert the following attribute inside the <a href...> tag:  rel="nofollow".

On the other hand, what if the placement is on a high ranking page that you want Google to know about?  (When your link shows up on a high ranking page, this improves your page rank in Google).  This is something you will have to test. to find out which weighs heavier - the negative impact of duplicate content or the positive impact of a high page rank link.

Happy optimizing!