Some companies used to set up multiple sites about basically the same thing, hoping to improve their search engine rankings. Some still do. Even if different products are promoted, using multiple domains is no longer a good idea, if it ever was. It is becoming more and more essential to adhere to the search engine's rules to avoid handicapping your rankings.
So if you have multiple domains and related content you should combine them into one major site and then use redirects to present one main web address to search engines. The best way to do this is a 301 “moved permanently” redirect. Using this code on your server lets the search engines know that there is only one main web address to spider.
As an example, if you have two domains, move your content from the secondary site to the primary site and use a 301 redirect to point the secondary url to the main site url. If you have multiple domains, choose one as the main site, one as the permanent redirect site, and then have all the other domains forward to the redirect site, which will then use a “moved permanently” redirect to the main site. Bruce Clay offers more details here.
