Some purists will tell you that having more than one domain is never a good idea, because you're just diluting your content or presence across multiple domains. That's probably true for content, but the reality is that it's not uncommon for businesses to have more than one domain name. They may have arisen from previous bad decisions or no-longer-relevant projects or trading names or whatever. Often there's good reasons to get additional names, like most australian businesses want a .com.au TLD, but then it's nice to own the .com too just incase someone types the address wrong or whatever.

Anyhow, I use [cloudns][1] for all my DNS records. They have a feature called "cloud domains" whereby you set up a zone for your domain, and then you can add as many cloud domains which will simply mirror the primary domain.

That's great for keeping things tidy, and avoiding too much messing around to make sure emails don't get bounced and that stuff, but it's not so great for SEO. The problem is, if you have one primary and multiple cloud domains, then from a user's (and google bot's) point of view, they can access your content at any of those domains. This is where you run into the problem I mentioned at the top. In the short term it may not be a big deal, but it's certainly possible that someone would find your content at one of your non-primary addresses, and share that link somewhere else. Once that happens you're diluting your presence between multiple addresses, which ain't good.

So..

[1] : http://cloudns.net "cloudns website"