Ask HN: What's the best way to implement multiple oauth authentication?

My team is working on a new project and we wanted to implement Facebook and Twitter OAuth verifications on top of the traditional email and password verification. We looked around on the web and setting up access keys to Facebook and Twitter are both pretty easy to figure out.

My question is, if a user registers via Twitter, adds some data to the system, logs out; and some time later decides to register via Facebook, how would the system know to combine those two separate registrations into one account? Is there a best practice for this exact problem already posted somewhere on the web?

Thanks in advance!