Jack O'Neill was an android created by Harlan in the physical image of Colonel Jack O'Neill, along with doubles of the rest of SG-1.
Biography
1998
The team was forced to remain on Altair and the duplicate O'Neill told the original one that they would bury the Altair Stargate. (SG1: "Tin Man")
2001
Instead, O'Neill's double demonstrated that, in Major General George S. Hammond's words, he was as good at following orders as his "real" counterpart. He and the rest of the duplicate SG-1 went on missions themselves, until they were captured by Cronus on Juna. The original SG-1 came to the rescue and, with the doubles, managed to save the planet, but every member of the robot SG-1 was killed in some way. (SG1: "Double Jeopardy")
Apocrypha
In the novel Stargate SG-1: Hydra, the NID acquire the android construction equipment from Altair, along with the readings taken of SG-1, and use them to create their own duplicates of SG-1, in an attempt to create their own offworld teams that will do what needs to be done. Although the original, 'Alpha' team is essentially SG-1 with certain crucial information- such as the address for Earth- deleted from their memories, later teams were created to increasingly diverge from the original to create a team that would do the job the NID wanted, culminating in the creation of the 'theta team', a team consisting of ruthless versions of SG-1 based on the Goa'uld memory implants acquired from Junior, the theta versions of Carter, O'Neill and Teal'c being described as psychopaths interested in nothing more than their own survival. Theta Carter attempted to use a device based on the Quantum Mirror to perfect a new power source despite the potential damage the device could do to reality, but she was eventually destroyed by the android O'Neill when she failed, Theta O'Neill being apparently wiped from existence when his attempt to use the power source caused quantum ripples in reality. The 'gamma' version of O'Neill- one of the last androids remaining at the NID project- eventually allowed himself to run down after destroying his base and the NID officials so that nobody could use SG-1's duplicates in that manner again.