Hi,
You should not use multiple domains/host names for the same site as this hurst your SEO. See the article Forcing a Preferred Host name. However if google has already crawled both domains you don't have control over which one they return in search results. By forcing a preferred host name you can make sure they only crawl the preferred domain in the future then they will gradually correct the search index. mojoPortal automatically creates canonical url links but if you are allowing more than one host name then it doesn't work correctly because the different requests would each have the hostname of the current request for the canonical. So it is important to force a singlke preferred host name. Then after a while when google has crawled again and updated their index it will start only showing the one domain.
Hope that helps,
Joe