PersistentId / principalName

[Peter Schober]

* Ortner Nikolaus <N.Ortner at fh-kaernten.at> [2012-07-05 10:00]:
> Sure, a blind update may not be ideal - maybe the IdP's
> StoredIDDataConnector should rise some kind of alert if the stored
> principalName shows a discrepance from the principalName given by
> the login-context.

I must be missing something rather obvious: How would the IdP
differentiate the case of someone else logging in from the same user
logging with with a changed userid?
-peter