Possible merging of two Apple ID's.

My mom had bought the iPhone 6s about 3 years ago, and after two months of usage, she gave it to my dad (who was using iPhone 5 at that time) and bought iPhone 8, I dont know but during the process of transferring the data,maybe their Apple ID's merged or something. For a long time, they both had access to each other's contacts. I was able to stop that, and things went smooth untill my dad bought apple watch s4, and while setting it up, at one point, it asked for the password of my mom's Aple ID. Yesterday, my dad bought iPhpone 11, and after setting it up, when all the apps were downloading, it stopped and asked for my mom's Apple ID. i have checked in the setting, in Mail, in iCloud, everywhere there is only one Apple ID, which is my dad's. Yet it asks for my mom's, whenever I try to sign in into a new apple device with my dad's Apple ID. It's frustrating, and this is the last place I can possibly go, so yeah, hoping for the solution ASAP.