In this episode, Bad Janet is revealed to have been disguised as Good Janet for a while, and yet in Janet and Michael (2017), it was shown that when a Bad Janet tries to be good, they would melt. However, this was explained by the fact that Bad Janet had been rebooted over and over, thus allowing her to evolve much like Good Janet and Derek had been shown to.
Once the gang discovers and interrogates Bad Janet in disguise, Bad Janet explains how she got there. In the visual recap it's shown that Bad Janet marbleizes Good Janet to take her place. However, Good Janet was marbleized via her right ear even though it's supposed to be the left ear, which was explained back in S2:E6 (Janet and Michael).
Eleanor, Jason and Tahani have trouble determining if Michael is the real Michael, or Vicky in a skinsuit. However, all they would have had to do was to ask him about something that they had experienced with him while Vicky was not around, and therefore would not have any knowledge of.