I've been following the discussion about Google and mirrored
information for some time. It is "common knowledge" that Google
penalizes page rank when it determines that content is
duplicated somewhere else. In fact, I've read many experts
stating that there should be no duplicate domain names and no
duplicate content anywhere.

On the face of it the arguments appear to be sound. Google
obviously has several billion pages in it's database and could,
it appears, easily determine if content is duplicated. It also
seems, again on the face of it, that it's reasonable to check
for duplicate content, as this is the "mark of a spammer" and
not necessary on the web with hyperlinking available. At least,
this is the common wisdom.

However, sometimes what seems reasonable and possible is not:
not by a long shot.

Let's begin with the technical side of things. You've got
domain x and domain y with exactly the same content. How on
earth would Google be able to figure that out? Let's say Google
had 3 billion pages in it's database. To compare every page to
every page would be an enormous task - quadrillions of
comparisons.

Now, if site x had page "page1" which linked to site y which
also had "page1", then it would be possible for Google to
determine the duplicate content. Conceivably, it could check
this out.

Not only is the task enormous, but the benefit is so tiny as to
be insignificant. Duplicate content does not imply in any way
shape or form spamming. In actual fact, a duplicate site is
generally going to lower page rank of BOTH sites. Instead of
having 100 links to one site, there will presumably be 50 links
to one and 50 to another. This would tend (all things being
equal) to lower the page ranking of both sites. So Google gains
nothing by this incredible expenditure of resources.

There are several reasons for duplicate content which have
nothing to do with spamming. Sometimes the content is actually
duplicated, and sometimes it's just that there are several
different domains (at least the www and non-www versions) for
the same website

Mirroring a site for load balancing - This is very common. The
purpose is to split up the traffic between two copies of the
site.

Mirroring for region - Sometimes site mirroring is done simply
to make it more efficient on the internet backbone itself. You
might put an identical copy of a site in Europe, for example,
to reduce traffic across the Atlantic, which should make it
faster in European countries.

Viral marketing - It's extremely common to allow other sites to
republish articles in return for a link.

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.