Cosima doesn't register the familiarity right away. Or, to be more accurate, she does, but she dismisses it. She spent the first few weeks imagining she saw people she knew, who turned out to be strangers when she got closer or they spoke. Nothing supernatural about that; she'd experienced the same phenomenon when she went to undergrad. It was just the brain making more connections than it should.

So as she comes up behind, she starts to say cheerfully (but quietly, because it is a library), "I wish you better luck with the existing classification system than I..."

But then she's close enough that she can't talk herself into the resemblance being coincidence, and her throat seems to have shut itself off.