Cloning is the process of creating a population of duplicates of a living thing down to their genetic structure. Naturally occurring in bacteria, plants, and even insects, more complex lifeforms like humans require cloning to be used with biotechnology to create copies of DNA, cells, and even complete living organisms.
Cloning has been used in several fields including on Earth where the most famous example being Dolly the Sheep along with several other species. Human cloning however was considered controversial and thus did not receive much research. On more advanced worlds like La Metal however, cloning is used as a more efficient means of reproduction often using genetic manipulations to make a few subtle changes to a person to better suit certain conditions. The reason for this is because La Metal leaves little time for traditional reproduction. The technology used for it seems to have spread. A notable example in some of the Galaxy Express 999 films is that Maetel's current body is a clone of Kanae Hoshino hence their similar appearances.