I've got some different (but kinda related) domain names that I forward to my regular domain address. I do it as a matter of course because...someone once told me it was a good idea for increasing search engine results.
Is there any advantage in doing this?
I also have eagleimagery.com which is duplicate content but I've been told that search engines will penalize me for it!
I'm not sure SE results are increased by doing this. Search engines rely on content for ranking. Redirecting does not give them anything with which to work. My rule here is that you only register additional names if:
1) you think there's a reasonable chance someone may type in another spelling of your name,
2) you use a hyphenated name and want the un-hyphenated version or vice versa, or
3) you registered multiple extensions (.com/.net/.org/.biz/.info/.tv etc.).
Otherwise, there are very few reasons to registers multiple domain names. I have one client that operates under its actual name, but uses a different (equally relevent) name for marketing purposes, and its hypenated version. This is rare.
As for the redirect itself, it should not be done through a metatag. Some browsers do not accept meta redirects. Some allow the user to disable them. And from what I've heard, many search engines punish sites that use these. The punishment may only apply to preexisting rankings, but it's still a bad idea. Use the redirect argument instead. Much cleaner, and much more search engine friendly.
Mirrored sites should be just fine, but every SE has it's own rules, so check the major ones to make sure you're not breaking them. Google is pretty good about making this info available. Don't know about the others.
Redirecting an address won't hurt you. If you have the domain registered and it doesn't cost you anything to redirect it, go ahead and do it, on the off-chance someone uses it instead by accident. Just make sure you perform the redirect at the server, not on the index page itself.