To be fair, I reckon Fern would have actually detected her own clone. She's also quite a systematic person, so I doubt she would have simply neglected to consider the possibility entirely. Not only that, but who wouldn't think of their own clone? That ought to be the first thought in anyone's mind.

How about that one dude who's always using a dublicate to avoid danger? What would Spiegel clone? A clone of a clone? I wonder if it's possible to draw all the relevant information out of a dublicate in the first place. Maybe his clone is just standing in a T-pose, unable to do anything, haha.