Why was Irene confused/surprised in the scene where she saw both Jerome and Vincent together?
In the scene where Anton comes to Jerome's house with Irene and samples blood from Jerome, Irene realizes that Jerome is not the person she knew (e.g. Vincent).
But didn't she already know Vincent was not Jerome before this? For example, in the scene where they run away from the detectives and hide in the alley, Anton shouts "Vincent!", and Irene whispers "Who's Vincent?". Also from subsequent scenes (bed scene, or when she helps Vincent by sending him back home from Gattaca), it's pretty clear she knows something is not right and Vincent is probably not the Jerome she knows.
Given all this, why does she act so surprised later when she finds out Jerome is actually someone else. Wouldn't she have expected that?