Yeah more then likely the code has already been redeemed, However it wouldn't hurt to try and redeem it. I have read a few topics where people bought used games and got lucky and could redeem the code. However if it doesn't work and you want access to multiplayer content you can head to the PS Store and buy an online pass for $10
online mode is free! you only need the pass if you want to level up over rank 15... I would have give you mine since I only needed the pass for online trophies. but I wasn't aware of 'free' online so I used it right away sorry.