If you own a web site AND you have multiple domain names registered, you might want to read this with your full attention. There are a great many web sites out there which, by inappropriate use of multiple domain names, effectively cripple their search engine rankings by blatantly breaching search engine guidelines!

Site owners often register more than one domain, and there are sound reasons for doing this, including;The main URL includes a keyword phrase but is too long to type quickly, so a shorter, cryptic one is utilised and maybe is also used on business cards and letter heads.You take various versions of domains to prevent competitors from grabbing themdot.com, dot.co.nz, dot.biz, dot.info etc, and point them all to the "main" site.You take various country versionsdot.co.nz, dot.com.au, dot.co.uk because you are going to expand into those marketsbut in the interim you point them all to the "main" site.

There are also misguided reasons for doing this, including;Having multiple domains pointing to the same location in the hope that the search engines will be duped into indexing ALL of them as different content, giving you lots of top rankings under different domains.

Search engines hate duplicated content with a passion' but their automated spiders and bots cannot determine which category YOU fall intouninformed and innocent of intent, or deliberately and knowingly manipulative. So all sites who transgress can expect to be treated equally. And yes, you can be banned from the SE indexes for getting it wrong!

The search engines do not give out many guidelinesin fact they are all very coy about giving too information much away. However, when they do make a statement by way of guidelines, regardless of how politely it is worded, you had best sit up and take notice because they are really, really serious about it!

Google Guidelines on Multiple Domains

In the case of Google, their Webmaster Guidelines are unambiguous! E.g. on this page; http://www.google.com/support/webmasters/bin/answer.py?answer=35769 Google are quite clear on this;
Quality GuidelinesSpecific GuidelinesDon't employ cloaking or sneaky redirects.Don't create multiple pages, subdomains, or domains with substantially duplicate content

What This Means to You

There is a clear and present danger in having multiple sites pointing to the same location if they load exactly the SAME content  this is a SERIOUS breach of Google's Webmaster Guidelines and is expressly FORBIDDEN because the search engines can only interpret this as a blatant attempt to generate duplicate content under different URL's. Regardless of your actual intentions in this, its likely to result in a BAN from search engine indexes. :) Not a good start to an online presence, right?

Ignorance of The Law is No Excuse

Ignorance of search engine guidelines is not a valid excuse either! Unfortunately, many web designers and hosting companies are blissfully unaware of the correct way to implement multiple domain names. The result is that many web site owners unknowingly jeopardise their site's inclusion in the search engine rankings.

My advice is that if you have more than one domain name, you check how its been set up. If your have two domains; www.domain-no1.com and www.domain-no2.com and BOTH load the same pages but with the different domains showing, then you have a problem. If you can also load the sites as http://domain-no1.com and http://domain-no2.com then totals 4 different ways of indexing the SAME content!

The Right Way to Do It 

That said, it IS permitted to have multiple domains pointing to the same content BUT it may only be done via the use of a Permanent 301 Redirect being applied to the secondary domains, e.g. typing in the URL for any of the secondary domains will redirect you to the main site. This change can easily be implemented, at no cost other than a few of minutes of time.

In this example, www.domain-no2.com is redirected to www.domain-no1.com and and neither visitors nor SE spiders ever "see" content under the secondary domain name. The same logic should also be applied to the non-www versions of both domain names to complete the process and to prevent potential conflicts with the search engines.