This question already has an answer here:
I have a (growing) problem with many of my sites. Someone, is creating mirrors of my sites (don't know if he does it only with my sites or others too) and replaces text with spam keywords. My SERPS has fallen more than 60% in the last 6 months and I think this is one of the main causes.
This site has copied 2 of my sites. It is cloaking the page. If you open the page on your browser you will see an ad but if you open it via google cache you get the spammy page.
How can I defend myself from this? Is there anything i could do? Maybe contact with google? Did this happen to you?