Full Idea
According to the 'mentalist' version of internalism, an externalist is someone who maintains that two people can be in the same present mental states while one has a justified belief and the other does not.
Gist of Idea
Externalism may imply that identical mental states might go with different justifications
Source
Hamid Vahid (Externalism/Internalism [2011], 2 A)
Book Reference
'Routledge Companion to Epistemology', ed/tr. Bernecker,S/Pritchard,D [Routledge 2014], p.145
A Reaction
It seems an unlikely coincidence, that we have identical mental states, but your is (say) reliably created but mine isn't. Nevertheless this does seem to be an implication of externalism, though not a definition of it.