“Smoke & Mirrors” is episode fourteen of season six of Stargate SG-1.
Senator Kinsey, who has been a pain in the SGC’s side ever since he first got them shut down in “Politics” (and nearly got the Earth destroyed by Apophis as a direct result), and who Jack O’Neill has threatened to kill thanks to his association with the NID, but instead blackmailed in “Chain Reaction”, is assassinated, and O’Neill turns out to be the biggest suspect. Although O’Neill claimed to be on vacation (with no witnesses), there is CCTV evidence showing Jack at the location of the shooting, and he is linked to the gun which shot the senator.
Thanks to the blackmail, O’Neill has no motive to kill the senator (even if he may have the desire to give him a good thumping) but this information cannot be revealed to the public. SG-1 is ordered by General Hammond to investigate the murder.
The murderer looks identical to O’Neill, but in “Foothold”, aliens invaded the SGC and mimicked the staff using some alien devices. Carter travels to Area 51 and discovers that all twelve of the devices have been stolen, and the thieves can as a result duplicate a number of the SGC staff.