Gemini

Gemini is the eleventh episode of the eighth season of Stargate SG-1.

Plot
The SGC receives an unscheduled off world activation, and with it an old IDC that had been used by Lt. Col. Carter. Then comes the message, "SEND MALP," and when one is sent through, it reveals a second Carter. She identifies herself as a human-form Replicator, designed by Fifth with all of Carter's memories and thoughts. Her message is urgent: Fifth has discovered a way to overcome the weapon O'Neill designed while he had the Ancient database downloaded into his brain. The replicator version of Carter has rebelled against Fifth and offers her assistance in altering the weapon to work again.

Carter accepts the proposal, and with O'Neill's approval, goes to the Alpha Site to work on the weapon.

However, it is eventually revealed that Fifth had no immunity to the weapon, and in fact he had sent her to study the weapon and develop the very cipher she claimed to be trying to defeat. Eventually, the replicator version of Carter betrays both the SGC and Fifth, taking her new immunity and escaping through the Stargate to continue building up the Replicators' forces.