Daniel Jackson was an android created by Harlan on Altair to have the physical appearance and consciousness of the original Dr. Daniel Jackson.
Biography
1998
The duplicate Jackson, along with doubles of the rest of SG-1, were left stranded on Altair with the duplicate O'Neill's promise that they would bury their Stargate. (SG1: "Tin Man")
2001
Instead, however, they went on their own missions to fight the Goa'uld. It was on a mission to Juna that, on Cronus' orders, the duplicate of Daniel Jackson was executed by Darian. When Darian noted that Cronus seemed as surprised as anyone else that the duplicate Jackson wasn't human, it helped convince him that Cronus was not a god. The duplicate of Carter tricked a Goa'uld into activating Daniel's self-destruct so she could escape. (SG1: "Double Jeopardy")
Apocrphya
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, to the point that the theta Daniel was described as a complete sociopath. Having learned about the truth of their creation, Alpha Daniel- who allowed himself to be called 'Dan' to distinguish himself from his template- contacted the real SGC to ask for their assistance in destroying the theta team. Theta Daniel was eventually shot by the original Samantha Carter when he tried to use Jack O'Neill as a hostage.