Local Account, Facebook, Google, Twitter Merge - Multiple Authentication

I implemented the passport.js authentication for Twitter, Facebook, Google and Local individually to my node.js application.

If user register himself with Local Sign Up and then in another machine if he wants to sign up with Facebook what should I do ? Another example, if user signup with Google+ and then in another browser/machine he would like to signup with Facebook, should I match the email ? What are the best practices for this purpose ?