It's possible to have two accounts on one site linked to the same account on another site, if the two accounts on the first site have two different OpenIDs and the account on the second site has both of those OpenIDs associated with it. For example, I made 620481 on SO. It has two OpenIDs, http://test1.mrozekma.com/ and http://test2.mrozekma.com/. I then registered 4833 on Unix with OpenID http://test1.mrozekma.com/ (which caused it to automatically associate with SO), and 4834 with OpenID http://test2.mrozekma.com/ (which also triggered an association with the same SO account). Both accounts show 620481 in their accounts tab as the linked SO account; SO shows 4833 as the UL account (presumably because it was associated first).
It's still a little unclear what problems this can cause; it was found in the wild on this PSE post