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This seems... suboptimal. Third party log-ins should be linked explicitly, not implicitly via verified e-mails. I was confused, looking for that option in the settings and not finding it... then realized it was automagic via email matching.
Something something email canonicalization. Here be dragons.
Does that assume that you can't change the email address on such an account? I've certainly seen services that do that and now I'm shut out from both Kobo and Ingress accounts... 
Good question!
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