Is this a good idea?
I'm learning polish using duolingo because I think it's just great and at the same time i'm using a textbook named "Teach Yourself Polish" so I can understand grammar. Is this a good idea?
Learning from both Duolingo and a textbook can't hurt, but some words and/or grammar may differ from each other.
That is a good thing. Just as native speakers of English do not always use the same words and grammar. It will give you a bit of enhanced learning.
Of course. The more resources you use to learn a language, the faster and better you will make it your own. Good luck! :)