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There are several possibilities. Possibly your competitor has copied your site and links to them from high-PR links. Google normally displays the site with the highest PR and sees other sites as copies. Normally you can check this by searching for "cache:www.yourdomain.com"
If it shows "This is G o o g l e's cache of [yourdomain.com...] duplicate content is not the problem.
If you want, sticky me the URL and the keyphrase and I'll have a look into it.
If this is your case, you can prevent the redirect when viewing these pages by placing the domain in your Internet Explorer's restricted zone (Tools.. Internet Options... Security... Restricted Zone). Make sure the settings for the restricted zone are default high security, or if you use custom settings for this zone, make sure Meta Refresh is disallowed.
I hope this is NOT the case for you, as it was no joy taking care of it. Today I just got the site shut down for the third time, after it popped up again on a webserver in China. Good luck.
If google continues to allow this... people with high PR could just duplicate the entire top 50 onto subdomains or new domains... it could be taken one step further... add links to known bad neighborhoods and get your competitors websites banned, this way when you are legally forced to take the site down, they will be banned anyway... or... sites that link to the original could be contacted asking to change the url they link to, once the changes are made the original site would have no PR.
I think Google should have a better way of handling duplicate content and protecting website owners from being at risk of this type of pirating. It seems like they should choose the site that has been in the index longer as long as it still has relavant content.