“Ripple Effect” is episode thirteen of season nine of Stargate SG-1.
The wormhole opens in the SGC with a strange noise, and SG-1 return from their mission earlier than expected. Whilst debriefing them, General Landry and the team see some odd differences between what their mission was, and what he thinks it was. Then the differences get bigger. Then the stargate opens again, and SG-1 return through it. Another SG-1.
The second team seems to be the genuine one. One possibility is that the first team came from an alternate reality, such as that entered using the quantum mirror encountered in “There But for the Grace of God”. The first team have come from a reality where the Ori are gaining ground. More ground than they are in this reality.
Then another SG-1 arrives. And another. And another. And so on, until the gate is closed to anything but emergency traffic. With enough teams there are differences in their members, and some of the teams see the return of long-dead faces such as Kowalsky and Dr Frasier.