I really have no idea. I didn't do anything special; rounded the corner, Psycho saw me, entered combat, won initiative, then hit with with some spells including doppleganger.
Earlier, in the arena during the psi beetle fight, they hit one of the other gladiators with doppleganger and it also wouldn't let me end combat. That time the doppleganger had the proper appearance though.
Testing it just now at the lake in DC and it doesn't happen.