Hi there,
I'm not playing this family anymore, but this exact same thing happened to me as well. I used the modifyrelationship cheat, (because I have quite a lot of sims in my neighbourhood and I just wanted to fix up the relationships of this household quickly), and instead of working as it has done in the past, it created a new sim with the exact same name and then changed the relationship with that sim to the amount I set in the cheat, if that makes sense. It sounds like the same thing lamia00 experienced.
I'm not that experienced with the Sims 4, so I honestly have no idea what could cause this. I'm not using any mods and don't have any expansions at all at present.
Anyway, I've moved on with my game for now, but I just wanted to let you know that I've experienced this too. All the best!