I'm interested in opinions on the best way to handle this problem:

You have a website which allows log-in via Facebook and Twitter alongside a regular registration system.

If a user logs-in via Facebook, makes posts, likes things, etc, then logs out and later logs-in via Twitter, the site won't know it's the same person and so they'll have blank profile with no posts, likes, etc.

What do you think is the best way to handle this situation and link/combine the accounts?