I'm unsure if I understand. Did two people get reincarnated into the one body of Lucia or was one personality the original NPC, falling insanely in love with the entitled girl for some reason (although her preferences should have been different as defined by the game)?
Lucia's body hosts two personalities: one is the original, in-game Lucia, while the other one is the selfish reincarnator.
The reincarnator was too self-centered to even notice OG's face, which was why she didn't even realize this; while the OG was originally programmed (written) to be a girl who loves those within her reach and in front of her eyes. In the game, it was supposed to be other people (hence her kindness) and the love targets (hence their romances), but since the reincarnator unknowingly kept her trapped in her mind, the only one the OG had ever seen was only the reincarnator.
As a result, she fell in love with the reincarnator instead, hence the yandere turnout.