6 ways competitors damage your site
Competitors can damage you in a variety of ways that can affect your rankings with are:
1. Inbound links from "bad" places.
2. Hundreds of links from one IP address.
3. Duplicate content:
4. from competitors scraping your entire site and creating their own from it
5. from competitors stealing your articles and submitting them to article submission services
6. 301/302 redirect hijacking (after all this time it STILL happens!)
More info on this site