Should SSO solutions find existing users by that person from other sites?

It's a social world out there and anything and anyone are expected to be connected up and with that the easiest way of having people try out your service is by having Facebook/Twitter/Linked In logins. The old 'password approach' is no longer such a feasible option.

But my question is: Do you feel that there is a need for multiple sign on solutions per user? If i have the option of signing in with Facebook and Twitter should the Twitter sign in automagically find my Facebook-created user and link those two up as well? I'm just wondering as it's something i have been annoyed at for a while.

Or is it fine that multiple signup methods will also create multiple users? The data returned from all these sites should have some sort of common data it should be able to find the others users by? PS: Sorry if some sentences seem incoherent, English is not my native language :-) and this was supposed to be a Ask HN post.