How do you solve the problem of users signing up/logging in with say their Twitter, but later also signing in with Facebook after logging out? Wouldn't there b

How do you solve the problem of users signing up/logging in with say their Twitter, but later also signing in with Facebook after logging out? Wouldn't there be a user account collision of some sort? Sorry if it's obvious, I haven't had a chance to check your site out. Good luck and wishes of great success to you.