A clone is a genetically identical copy of a living being.
Human cloning technology has long been sought after by LuthorCorp, resulting in several troubled cloning projects. These efforts culminated in Project Gemini, which successfully cloned the long dead Julian Luthor as Grant Gabriel.
Kryptonian clones have also existed, most of them created by cloning technology that the Kryptonians possessed.
The process of cloning was first developed under the supervision of Lionel Luthor, by the LuthorCorp geneticist Pete Dinsmore. Dinsmore first applied tested the process on a series of infant rabbits, which he was successful in aging to adulthood. With this success, Dinsmore attempted to clone his deceased daughter Emily, and then applied the experimental process to accelerate her aging so that she would be approximately the same age as the original Emily when she died. The acceleration process involved the introduction of Kryptonite into her biology.
However, the aging process resulted in severe side-effects. Specifically, Emily's moral center was damaged beyond repair, which resulted in her becoming a childhood sociopath with no internal concept of right and wrong. She also gained the ability to move at super speed. Lionel later abandoned the project after Emily murdered her father and it was discovered that Emily's accelerated aging would not slow down.
Lex Luthor later resurrected LuthorCorp's experimentation with cloning. LuthorCorp was able to successfully able to eliminate some of the side-effects which had plagued the previous project. Specifically, they were able to eliminate the damage to the moral center and the superhuman speed. 
Relocation of Luthorcorp Projects
At some point, Lex relocated the project to Cadmus Labs where he continued his experiments and kept it away from any suspicions.
Lex cloned his own DNA for the hypothetical case that he needed it in the future, and effectively, after he was badly injured during the collapse of the Fortress of Solitude, he uses this project to harvest his clones body parts and organs. As had happened before, the clones didn't turn out well and some of them were sick and aged fast. All the clones but one were destroyed when the rogue clone LX-13 put the lab on fire.
It was revealed later that Lex also used Clark Kent's DNA on his experiments and with the Kryptonian blood was able to counteract the side effects of the project, but only one of the clones was successful to resist. This clone later became Conner Kent.
Kryptonian scientists were noted for their great knowledge about genetic and conducted experiments based on this knowledge that led them to several successes and failures, one of them, the dysfunctional experiment known as Bizarro, who that after being deprived of its corporeal form for being extremely dangerous, kept the ability to clone the Kryptonian DNA in order to survive. He made a replica of Clark Kent on the basis of his DNA.
Leading scientists in this field were members the House of El, who managed to develop the project by far and succeeded in their experiments. They were able to make perfect clones requiring only minimal DNA structure and without specialized support machinery, such as in the earth projects, thus simplifying the complicated process. However, it is noteworthy that most of their projects involve Kryptonian crystals and artifacts that require external manipulation to act.
Survival of the Kryptonian Race
Driven by the Rao's prophecy, Kryptonian scientists focused on finding a way to ensure the survival of their race in the event that the prophecy was fulfilled, and this was what motivated them to develop the process of cloning.
Jor-El was in charge of this project and was successful in collecting DNA samples from the best soldiers and scientists of Kandor in the battle against Black Zero, coming later to store the samples within the Orb and send them to the earth to achieve the emergence New Krypton. To avoid a conflict with the human race, he deprived the clones of their powers, radiating the samples with blue Kryptonite.
Under the same goal, Zor-El took samples of his own DNA and that of Lara, and placed it inside a crystal to later instruct his daughter Kara to release them upon reaching on earth.
- Emily Dinsmore - A clone of Lana Lang's old friend that was empowered by Kryptonite.
- Model 503 - A clone of Lana Lang.
- Grant Gabriel - A clone of Julian Luthor
- Adrian Cross - A clone of Julian Luthor.
- LX-13 - A clone of Lex Luthor.
- Conner Kent - The hybrid clone of Clark Kent and Lex Luthor.
- Zor-El (clone)
- Lara-El (clone)
- Zod (clone)
- Kandorian army
- Jor-El (clone)