John Hunter was a United States Air Force Major stationed at Stargate Command and a member of SG-12.
When Hunter was a Lieutenant at Stargate Command, he was assigned to SG-12. (SG1: "Gift of the Gods")
Some years later, he was captured by Ba'al and replaced by a clone, which was then able to infiltrate and serve on the Sun Tzu as Colonel Yin's second in command.
The real Hunter was later rescued. (SG1: "An Eye for an Eye", "Infiltration")