What Is A Mirror Site?
The term "mirror site" refers to an identical website hosted on another domain/website address. Large websites usually provide a mirror website for different continents. This allows their users to experience faster download speeds because they're working with a closer server.
Why It's Spam
Although this is a perfectly legitimate practice, many search engines consider it spam, because the amount of pages is doubled, yet the content is identical. Most search engines use "Duplicate Content Filters" that should eliminate mirrors from distorting the search engine result pages, although in rare cases, one or all mirrors may be removed from the index of a search engine without any prior notice.