FAQ Answer:

Should "mirror sites" or "pages" be created ?

Answer:

Mirror sites are web sites that duplicate another web site.  Whether one has created a mirror site or just a mirror page, they are frequently frowned upon by some of the search engines and could cause a site to be penalized by that search engine.

An example is the following, which was is taken from

Open Directory (DMOZ) (FAQ)

"We care a great deal about the quality of the directory. To keep the ODP running smoothly, we may have to delete submissions that violate these policies or that we believe otherwise disrupt the operation of the ODP. In some cases, we may also delete other submissions by users violating these policies.

• Please only submit an URL to the Open Directory once. If a site has not been listed within three weeks, you may submit it again.

• Disguising your submission and submitting the same URL more than once is not permitted. Example: http://www.dmoz.org and http://www.dmoz.org/index.html.

• Please do not submit mirror sites. Mirror sites are sites that contain identical content, but have different URLs altogether.

• Sites with illegal content are forbidden in the Open Directory. Examples of illegal material include, but are not limited to, child pornography and sites infringing on copyright.

• Please avoid submitting sites with addresses that redirect to another address.