Has anyone been penalized for having mirror URL's setup for other popular domain names that are parked at your main website and do not include a URL redirect. I am just wondering because I have several other domain names parked on top of my main site, and some of them get a nice ammount of traffic and google crawls them as well, although it is the exact same content as my main name.

I've seen several large forums that have the same thing going on, where all the content is the same on multiple names and they do not include a redirect, just have the domain name parked on the main site with the same content under a different URL, and they don't seem to have a problem.

Is it dangerous to keep doing this, or have others done this and never had a problem? I mean if it is really something you shouldn't do, then I would have no problem setting up URL redirects for my other domain names. I just thought it would be strange and a long shot for a search engine to drop you or drop your results lower as many websites have duplicate .com, .net, and .org domains, or similar domains that do not use a redirect.