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 has two souls inside her: Original Lucia, and Isekai Lucia.
When Isekai Lucia came to this world, she basically kicked Original Lucia out of her body and took control over it. Original Lucia hung around as another "personality" within her, and she can only move when Isekai Lucia is not conscious. During those times, she made plans for Isekai Lucia's demise, like sending those anonymous letters to the Queen and Gerald in previous early chapters because she also gained Earth knowledge from Isekai Lucia's ramblings.
Basically, Isekai Lucia was fucked no matter what, even if Alexandra's plan didn't come to fruition.
In the end here, Original Lucia finally gained control of her body back because of how mentally and emotionally drained Isekai Lucia is, and as we see in this chapter, she has zero intention of letting Isekai Lucia out again.