mvc4 simple membership + Linked in Oauth

Hi there, am using asp mvc4 simple membership to allow users to register on my site but I now want to allow linked in oauth to create accounts. I like how stack overflow knows if someone on the same computer is trying to create 2 seperate accouts if they try register with a oauth mecahanism and also a standard register on the site. Can someone offer advice on how it knows this and instead knows to link the different registrations or logings to the same user.
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