Household all have same name

@andr99 If only surnames changed, that could be that your MC Command Center settings reverted to default. If first names also changed, I don't know of a mod that can cause that and I advise going to Game Issues or Bug Reports.

both the first and the surnames are changed, btw thanks I'll try going to bug reports 

I discovered this glitch while playing the Kahananui family household in Sulani. I went in to edit Alika Kahanui's turn ons and turn offs. I checked what personality traits his wife had, so I could easily make them attracted to each other and it showed that both household members had the same personality traits, which I thought was weird, but whatever. Then I started editing his turn ons and turn offs, and switched over to his wife to give her some, and then realized that both household members had the same likes and dislikes and turn ons and turn offs that I had just set and if I changed hers it would change his to match. I didn't even notice that both sims had the same name until I got out of CAS mode, and saw that. I noticed that while trying to figure out how to fix the problem in CAS mode, by editing her turn ons and turn offs and then switch back to him to change his back to what I said his were before, hers were actually what I had tried to set hers to in CAS, before I switched to change his back. Her personality was also different than his in live mode when I checked. It only would say they were the same in CAS. Then I started a new game, and attempted to make a two person household. The problem also happened here, and all household members would default to the same name, personality, and likes and dislikes. I've only tested this on adult/young adult sims though.

@SilverGrayce Is this happening and you have mods? Or did you have an empty mods folder when it happened?

this happened to me just now. I went to edit my teen sim who had just aged up. I needed to use cas.fulleditmode because I couldn't see the skin tone of her mother and her alien disguise was looking more Asian when her mother is Caucasian. once I finished editing I clicked done and returned to the game and noticed both the teen and her mother had the daughter's name. I went back to CAS and tried to change the mother's name back and it changed both names. saving and reloading didn't reset their names.

I exited and went to a previous save luckily I had saved shortly before aging the child. so I only had to edit her disguise again. 

**note: just editing an alien's disguise does not affect the names, but any entering of CAS causes the names to be the same for which ever sim you use to enter CAS. also exiting using the x (not saving changes) keeps their proper names.

I have never used any mods. I don't remember the last time I entered CAS so I'm not sure how long this has been happening.

I just remembered the last time I entered CAS, the fact my alien sim has a mother was my reminder... I did cas.fulleditmode to turn my sim into a male so she could get pregnant by abduction. then after giving birth I returned to cas.fulleditmode to return her to normal. when switching her, the clothing randomized so I needed to edit all of her clothing both times. when the alien child was an newborn the name didn't change. so either newborns aren't affected or it was fine a few weeks ago. 

@Sirromap To confirm, you have a completely empty Mods folder (except for resource.cfg) when this happens? And you're on game version 1.108, not an earlier version?