Mirror – SEO WIKI
A site mirror is a complete copy of a site hosted on another domain (not the main one), which may appear, for example, when changing the primary domain or to distribute the load on the server.
The number of mirrors can be any, but if the domain was glued together successfully, only the main one will be present in the search results.
The gluing and gluing of mirrors occurs with the help of a special robot-mirror, which operates on the site according to its own algorithm. It is impossible to predict its appearance, and sometimes it has to wait long enough.
Mirrors are not the same as site aliases. If the former are created to distribute server load or traffic across regions, the latter are simply synonyms of the main domain name.