The Problem
The Solution
A better option is to park the alternate domains on top of the main domain and create a search-engine friendly 301 redirect. Search engines will index only the new URL, but will transfer link popularity from the old URL to the new one so that search engine rankings are not affected. The same behavior occurs when additional domains are set to point to the main domain through a 301 redirect. By using what is called an "IP funnel", you will have multiple domains pointed to the same site but you won't risk duplicate content issues.
You can also use a 301 redirection for common misspelled versions of your domain name or for other good domain names that you don't want your competitors to get.
