A Site Mirror is a complete or near-complete copy of the content of a website, but accessible at a different URL or domain. Mirroring is typically done for purposes such as backup, maintenance, traffic redirection during high traffic times, or regional segmentation.
Conceptually, mirroring is a technical problem for search engines because it results in duplicate content. Search engines are forced to choose which of the two copies is the “main mirror” (the canonical version) and ignore the other, which can lead to a decrease in overall authority and confusion in rankings.
To manage this problem, SEOs should use canonical tags (rel=”canonical”) or 301 redirects to indicate to the search engine which URL is preferred for indexing.