how come no two pokemon are allowed to enter while holding the same items on the same side?
Because that's cheap. Would you want to go up against a team in which all 3 pokemon were holding a focus sash/band (whichever is more effective) when you're on the 48th battle; one away from the 2nd battle with the tower tycoon? No, didn't think so.
^ Anastasia-R ^ Current VPP: Palkia > Level 100: 6987