I’m experiencing this problem with my mom and grandmother merging. It’s not because of their Apple ID being the same because grandma doesn’t even have an Apple phone. What happened is my mom sent a text of my grandmas number and copy pasted it from her messages. It said “this is my number” so Apple got confused and added her number to my moms. Ive deleted that message and I’ve fixed both of thier contacts and made sure they’re seperate but grandmas texts are still in my mom’s thread. I can’t text my mom or grandma anymore without texting both at the same time. please help!