Using Multiple Domain Names - from Matt Cutts of Google

Matt Cutts, the head of Google's Webspam team talks about using multiple domain names:

If the content is truly the same, I would pick one domain and make the other domains do a redirect to the domain you prefer. For example, Google.com could do a permanent (301) redirect to www.Google.com, and then we would see that and generally choose the destination of the redirect. Having content from two different domains isn't risky if they are in different languages (for example, Chinese and English), but if you have the exact same content on two different domains, it's better to use a permanent redirect from the duplicate domains to a single preferred domain.

If you have mirror pages without a redirect, that can cause issues. It's better to use 301/permanent redirects, because Google might choose to remove or not to show the copy that you want to be seen most.