when does peter find out about alternate olivia
When she fails to recognize a particular phrase.
Entrada (Fringe)
At the end of the previous episode, Olivia Dunham (Anna Torv) was able to cross over back to her universe long enough to warn Peter Bishop (Joshua Jackson) she is trapped in the parallel universe. After receiving the message, Peter tests the Olivia from the other dimension, "Fauxlivia", by telling her the Greek phrase "Na einai kalytero anthropo apo ton patera tou" (roughly, "May he be a better person than his father"), which Olivia told Peter in the "New Day in the Old Town". When Fauxlivia fails to recognize the phrase, Peter confirms his suspicions. Realizing she has been exposed, Fauxlivia forces Peter to inject himself with a paralyzing agent. She then goes to a typewriter store in the Bronx to contact the parallel universe to request an extraction.
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Entrada (Fringe) [SEP] Plot
At the end of the previous episode, Olivia Dunham (Anna Torv) was able to cross over back to her universe long enough to warn Peter Bishop (Joshua Jackson) she is trapped in the parallel universe. After receiving the message, Peter tests the Olivia from the other dimension, "Fauxlivia", by telling her the Greek phrase "Na einai kalytero anthropo apo ton patera tou" (roughly, "May he be a better person than his father"), which Olivia told Peter in the "New Day in the Old Town". When Fauxlivia fails to recognize the phrase, Peter confirms his suspicions. Realizing she has been exposed, Fauxlivia forces Peter to inject himself with a paralyzing agent.