At the moment competitors can damage your site in several ways, things that Google lets your competitors affect your rankings with are:
1. Inbound links from “bad”/”dodgy” places.
2. Hundreds of links from one IP address.
3. Duplicate content:
a) from competitors scraping your entire site and creating their own from it
b) from competitors stealing your articles and submitting them to article
c) from sending bogus query strings to your URLs
d) getting your site spidered under SSL
4. 301/302 redirect hijacking (after all this time, it STILL happens!)
and there is new 301 hijack which is causing sites to drop like flies.
People who do this are scum to say the least.
Watch out and stay safe.