Cloning is a process from which the result is an organism whose genetic information is identical to that of a "parent organism" from which it was created. The primary problem caused by cloning is every subsequent copy becomes less and less precise. Like a copy of a copy of a copy will tend to lose the original's high definition qualities, so too does the clone of a clone of a clone.
The Asgard reproduced exclusively through a process of enhanced cellular mitosis, and through cloning had achieved a measure of immortality. Asgard DNA was programmed to grow a clone to maturity in just three months, however the clone was essentially an empty shell until an existing consciousness was transferred into it. As each Asgard's body failed, his consciousness was transferred into a newer, younger version of himself; thus, Asgard such as Thor or Freyr—who were known to have been early humanoids—had survived for thousands of years. However, lack of genetic diversity had led to eventual genetic deterioration, and the Asgard had become a dying race. Also there were certain degradations due to making a copy of a copy instead of copying the original body, which occurred.
Asgard scientists—such as Heimdall—had attempted a process of controlled mutation to stave off genetic degradation and preserve the Asgard race, but eventually failed. Loki had attempted less scrupulous methods, performing unsanctioned experiments by abducting humans from Earth and replacing them with clones for several days so as not to arouse suspicion while studying the abductees before returning them. Loki created a clone of Colonel Jack O'Neill, but the clone failed to age properly, and the new Jack O'Neill emerged as a teenager with a short life-span until Thor repaired the error in his DNA, allowing the clone to grow at a normal rate. Eventually, however, the Asgard had made too many irreversible mistakes in their development, and the final attempts to solve their physiological degeneration had left each of them with a rapidly progressing disease. As a race, the Asgard planned to end their lives in a mass suicide before their degeneration and suffering progressed too far, and they destroyed themselves and their homeworld of Orilla. (SG1: "Revelations", "Fragile Balance", "Covenant", "Unending")
Vanir cloning technology in Pegasus beared significantly improved results thanks to experimentation on humans. They appear to stave of degradation itself though final solutions still evades them. (SGA: "The Lost Tribe")
Ba'al took advantage of genetic replication technology when he took refuge on Earth following the defeat of the Goa'uld at Dakara. By cloning many dozens of copies of himself, including several who remained on Earth, he became a formidable and elusive enemy to his adopted homeworld as the Tau'ri repeatedly encountered and eliminated his duplicates. Clones of Ba'al were killed by Gerak on Dakara, in order to make the Jaffa think he had been eliminated, by Teal'c after his kidnapping on P2M-903, by the Lucian Alliance on his Ha'tak during a plot to steal Stargates, by Stargate Command as nearly two dozen clones attempted to escape from the base, by Adria's warriors near Merlin's cave, by his own hand during a summit on P3R-112, by SG-1 during the capture of Adria, and his symbiote died during surgery to remove it from the host body of Adria. His final clone was captured and the symbiote was extracted from the host and died of the extraction, with the original Ba'al being killed in the past by an alternate version of Cameron Mitchell. However, the host of the final Ba'al clone survived. (SG1: "Ex Deus Machina", "Stronghold", "Off the Grid", "Insiders", "The Quest, Part 1", "The Quest, Part 2", "Dominion", "Continuum")
Eurondans were made up solely of cloned individuals of unknown number of original persons. They used this method in order to preserve what they saw as genetic purity. The majority of their clones were kept in stasis, where their heart rates were kept at an extremely low rate.
Scientists on Earth have also made some successful attempts at cloning technology. Colson Industries was able to grow an Asgard clone from a DNA sample in a few months. Hoping to create a miracle drug, scientists at Immunitech Research had successfully cloned a Goa'uld symbiote before it was implanted into Adrian Conrad. However, a lab accident allowed the cloned symbiotes to take hosts among the population of Steveston, Oregon, where they became known as "nightwalkers." (SG1: "Nightwalkers", "Covenant")
The Wraith also made use of cloning technology in their war against the Lanteans in the Pegasus galaxy. Seeing their enemy possess such advanced technology, the Wraith realised that they needed an advantage if they wanted to win the war. After luring three Ancient warships into their domain, they managed to disable the vessels and take the Zero Point Modules that they were equipped with. Using this potent power source, they created a secret outpost where they created hundreds of chambers in which thousands of clones could develop.
The cloning process involved a Hive Queen to create several Wraith warriors naturally. Once these "templates" were created, the machines would duplicate the gene patterns of those Wraith several thousand times creating an instant army. It was through this method that the Wraith managed to flood the Lanteans with pure numbers that allowed them to win the war. Todd told Dr. Rodney McKay that "Within weeks, our army had grown to hundreds of times its original size. From that point on, the tide of war turned in our favour and there was nothing the Lanteans could do." Todd attempted to employ this tactic 10,000 years after the war in order to defeat the rival Hive factions. This plan failed due to the actions of a rival Hive Queen and the Atlantis expedition. The clones are somewhat primitive and are used as grunts by the more advanced Wraith that are naturally born. They usually wear a distinctive mask. (SGA: "Spoils of War")
Michael Kenmore also had access to cloning technology. Whether this is Wraith technology or that of other races is unknown. Michael used the technology to clone Dr. Carson Beckett. Clones created by this technique need to take shots of a serum regularly otherwise they die of organ failure. It is possible this is the same cloning technique used by the Wraith, as the Wraith's regenerative abilities might make the serum unnecessary. (SGA: "The Kindred, Part 2")