Why can't I login in to mail on my new iphone 6S?

I just bought the new iPhone 6S. I set it up as a new one but I still have my apple-id etc.

But here comes the problem: I can't login to my mail account in mail. It says that my account already have been added to iPhone. ??

What am I supposed to do? Need help ASAP! 🙂

Thanks!