Duplicate and mirrored content
(web browsers) Bar Graph MDI
(web browsers) Headlines MDI
(web browsers) Pioneer Report MDI
(web browsers) Tree MDI
GRML web browsers
(web browsers) Bar Graph MDI
(web browsers) Headlines MDI
(web browsers) Pioneer Report MDI
(web browsers) Tree MDI
(web browsers) Bar Graph MDI
(web browsers) Headlines MDI
(web browsers) Pioneer Report MDI
(web browsers) Tree MDI
(web browsers) Bar Graph MDI
(web browsers) Headlines MDI
(web browsers) Tree MDI
(web browsers) Pioneer Report MDI
Different domain names - Sometimes a site might be referenced on many different domain names. You might want to allow the .com, .net and .org versions of the name to all work the same, you might allow for common misspellings or you might cover different keywords (sewing-tips and sewing-secrets are examples of possible combinations).
Different domain names for different markets - you might also want to reference your site by different names in order to target different markets. You could, for example, have a site about search engine optimization and want to target both SEO and web designers. Thus domain names like seo.com and webdesign.com would make sense.
www - Any good webmaster knows his or her site needs to be referenced with and without the www.
Okay, so what's the smart thing to do? Well, it is possible that search engines do compare a limited number of pages to check for duplication. They could certainly check if someone reported something, and they might check directly linked pages (although this is still a heck of a lot of overhead for very little benefit).
Of course, Google and the other search engines can account for a hefty percentage of the traffic received by a site. In fact, sometimes the number can exceed 70 percent. So it's wise to spend some time ensuring that you are totally clean when it comes to search engine optimization. In other words, a technician from any search engine should be able to examine your site down to it's smallest detail and find no evidence of any kind of search engine spamming (attempting to get higher rankings by unethical means). This is absolutely critical to a site's survival for the long term.
Keeping that in mind, here's what I tend to do.
Multiple domains - Using multiple domains to the same site has a tremendous number of advantages. Thus, I tend to follow the advice given by others: take advantage of permanent redirection. In other words, set up a redirection (a 301 status code) which simply tells the browser "this page has moved, proceed to this page, and the move is permanent. This tells the spider about the redirection with no possibility of misunderstanding, yet allows for the multiple domains.
Republished articles - I allow others to republish many of my articles, and at this time I have records of over 10,000 of them all over the internet on thousands of web sites. This is not a problem, as these articles are sent in text format. The webmaster must then drop this text into his site, which requires some reformatting and shuffling around. Thus, the finished articles may have the same text but the formatting is very, very different. This is a highly respected method of gaining a large number of incoming links: I give you something (an article, i.e., content) and you give me something (a link back to my site).
Mirroring - I haven't needed to do this yet, so I have no advice as to what to do if a site requires actual, physical multiple versions of itself. I would tend to just do it overtly (out in the open) and not worry about it"