I have always thought that having multiple domains point to one site could hurt your rankings. I was just curious if there are any "special practices" to follow when doing this to make sure you do not get penalized by Google for duplicate content? Is my belief no longer relevant?

Do you mean multiple domains hosted on the same server?

multiple domains on one server? I'm not sure what you mean. Maybe I should explain my situation:

I have a client who wants to purchase multiple domains (in attempts to capture a wide audience) and point all the domains to one web site, example-sales.com. I've seen people do this before, but I just want to know if it hurts your rankings because it is duplicate content? Doesn't Google penalize you for doing this? Are there other considerations I should take into account?

Multiple domains pointing to the same site is fine as long as you 301 rewrite them to the main domain so that Google doesn't find duplicate content.

Thank you. After Googling this subject for a while, this is what I found as well.

What doesn't make sense to me is that part of Google's pagerank algorithm uses the url as an indicator. So if you tell Google not to index the domain because of duplicate content, aren't you defeating the purpose of purchasing that domain?

The only real reason to have additional domain names in my opinion is that it's cheap insurance that someone else won't get that domain name and start competing with you. No matter how many domains you have hooked up to the same account, you only want one to actually answer.

Another reason for having more than one domain hooked up to an account might be that (ie) companyA might have had a website for a long time with the domain name companya.com. But they realized that since they're selling widgets, a better domain name to rank for widgets might be widgetscompanya.com. But everyone already knows their email addresses are users@companya.com. Hooking up the widgetscompanya.com domain and having only it answer would still allow them yo use their original domain email addresses.

From a technical and SEO standpoint:
Sometimes you might want to duplicate the content on all domains, pointing them to the same documentroot.
If you want to user to think he is on the website name he is on, use php to detect the domain and change the branding accordingly, but if it's googlebot, redirect it to the 'main' website (as google sees it).
If you want the viewer simply redirected to another website (which is typically best for SEO, along with the last solution), you can use an .htaccess script to do that.
A 301 redirect is the best redirect for this situation, when the site is 'moved permanently', while a 302 is temporary, which isn't as good to use for seo.
If you can't get apache to redirect, a 1-liner php script can do the same:
Saving that as index.php will redirect the domain to another.

You also can redirect child pages (like http://shop.com/products to http://shopnow.com/products) using apache or php with slightly more code to find add the rest of the url to the redirected page.

If you concentrate over one domain name and site and then spend time and cash promoting that you could achieve better results.

Look at it this way. You have one mother web site with several children. I call the children "sister sites" because they are essentially performing the same duty; empowering the mother.

In a perfect world, for best SEO, each sister site would have indisputably unique content and each have a significantly different marketing angle (keyphrase targets). In this model, there's no need for redirect; you make best use of each domain. One can develop a powerful linking structure this way, effortless satisfying Google's off-site factors by concentrating on your web pages' on-site factors and how pages link between domains. You are in a position to make something really strong and sustainable. Don't waste your domain real estate by redirecting it.

To use the same content repeatedly across all the domains offers no additional value for either the Internet visitor nor the search engine. You're lucky, you have the tools, may as well capitalize on your opportunity.