A client had been using their parked domain as their primary URL. For all intents, this presented no problems. However, after months of difficulty getting traction in Google SERPs, it became very apparent that it was creating problems. The reason: duplicate content.

Most search engines don't care particularly much about duplicate content. From research, it appears that Google considers parked domains as duplicate content and penalizes these sites in the rankings. Althougt most domains (xyz.com / 123.com) do appear to get 'merged' after awhile, for sites that want to park multiple domains, one of two things should be done.

1) Creates two separate accounts: the main domain account and an account that supports only the 'parked domains'. On the parked account, use a 301 redirect to the main site. Supposedly, this tells the Google that the URL is permanently moved and passes the duplicate content test.

2) If you are utlizing cPanel, add an 'Add-on Domain' of the domain you would like to park. Then, setup a redirect to the index of the main site. This creates as similar 301 permanent redirect that passes muster as non-duplicat content.

In both cases, one should utilize a Header checker to gauge what the Spider sees. Keep in mind that this technique has been known for sometime and comes up regularly.