I get that Duroa can seem overly paranoid, but think of things from her perspective: She has potentially been brainwashed by Nocton for nearly her entire life. She literally can't trust her own feelings or perception around him because he could be manipulating her constantly. While we, the readers, know what's actually going on . . . she doesn't, and Nocton has been terrible at explaining things to her. Just looking back on her life, it's clear that she was unusually attached to Nocton despite being treated like shit.
Basically, she has every right to be paranoid. The only real way to start clearing things up is for him to call her by a different name.