Why would Pollux suspect the guy looking like his brother wasn't his brother?
I mean yeah when Castor (really Archer) first arrived at the prison he was acting a little odd and didn't smash Dubov's face in but honestly what real reason did Pollux have to suspect that this guy was anyone but his own brother? he had no reason to even believe face swapping surgery was even a thing.
Also what ended up convincing him the guy was his brother? Was it the comment about his medication? Or the fact that he voluntarily brought up that he didn't know why Dubov jumped him (which played into his story about his memory being unclear due to the concussion and the coma)?