In which episode is this scene with two Zoros?
This appears to be an animation error from an earlyish episode when that was more frequent.  Specifically it comes from Episode 226 which is part of the Foxy Pirate Crew Arc.  

In this scene Zoro is fighting Foxy the Silver Fox in (a bad) disguise to the left hand side of the screen and also (apparenly) calmly standing there watching the silly fight.