With 12 strings it's generally advisable to tune it a whole tone below standard tuning. I used to own a Yamaha 12 string and got tired of using a capo all the time and started tuning it to standard. In less than a year the bridge had bowed up and the guitar was useless.

If it's already happening to your guitar I'd return it if I was you. It's not going to get any better and it'll just make it harder to play.