How to prevent duplicate user accounts with Facebook and Google authentication?

Hi,
I'm interested in completely "outsourcing" the authentication business to Facebook and Google in my MVC 5 app. The part where I'm confused is this: I'm sure a lot of the users have both Facebook and Google accounts. How do I prevent duplicate user accounts in my application for users who may first use Facebook then Google -- or vice versa -- to authenticate? If a user comes in and authenticates using Facebook, I create a user account for the user in my database. The next time he/she comes in and uses Google authentication, how would I know that this user is already registered in my system and not create a new user account?