Cloning technology uses genetic material to create an exact duplicate of a living organism. 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 early cultures of humans, 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.
Asgard scientists such as Heimdall had attempted a process of controlled mutation to stave off genetic degradation and preserve the Asgard race. 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 O'Neill, but the clone failed to age properly, and Young Jack O'Neill remained a teenager until Thor repaired the error in his DNA. 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 sort of mass suicide before their degeneration and suffering progressed too far, and they destroyed themselves and their homeworld of Orilla.
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 as SG-1 repeatedly encountered and eliminated his duplicates. Clones of Ba'al were killed by Gerak on Dakara, by Teal'c after his kidnapping on P2M-903, by the Lucian Alliance on his mothership during a plot to steal stargates, by the SGC 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. SG-1 cannot be certain that they have seen the last of Ba'al and his clones.
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 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."
Genetic research among rogue elements within the NID led to the creation of Anna, who was at first suspected of being a clone, but who was, in fact, a genetic hybrid created by splicing a human ovum with Goa'uld DNA. Although the duplicate SG-1 team created on Altair by Harlan have been referred to as clones, they were in fact synthetic duplicates, robot bodies created to hold a copy of a human consciousness.
Cross Reference: Anna, Asgard, Ba'al, Alec Colson, Adrian Conrad, Heimdall, Loki, Nightwalkers, Young Jack O'Neill, Pamela, Synthetic Duplicates, Werner, Zyang Wu
Episode Reference: Revelations, Nightwalkers, Fragile Balance, Covenant, Ex Deus Machina, Stronghold, Off the Grid, Insiders, The Quest, Dominion, Unending