My contacts have merged

An AppleID is meant to be a single person’s unique personal identifier for all Apple services (iCloud, FaceTime, iMessage, the App, iTunes, iBook and Mac stores, these forums,...). They were never intended to be used by multiple people, and with the increasing number of features using iCloud to sync data and content across devices, sharing an AppleID will simply cause ever more headaches and problems since the data is not separated by user (the system sees one user = the AppleID signed in).

Quite bluntly, the only solution to your issue is for everyone to use their own AppleID. Otherwise, you’re all using a single integrated account for every Apple service you use. As Apple introduces even more iCloud based sync services and features, your issues will just get worse and worse as you continue to share a single AppleID.

Apple is is not going to change anything about this as their system is working as they intend it to - where an AppleID identifies a single unique personal login ID.

Use family sharing to share content - Family Sharing - Apple Support

Does she have imessages in both devices set up to use the same address or telephone number? Is she using messages in iCloud to sync messages across all devices?

I have two Macs, three iPads and two iPhones and all my messages always appear on all my devices, since I only have one AppleID and I use messages in iCloud. They also all are setup for the same “receive at” number and emails.

And it is indeed the case that one AppleID does equate to one single account for each Apple service setup and signed into with that AppleID. It has always been that way.

Messages are not set to sync across all devices. iPad is set to receive messages at email address. iPhone is set to receive messages at phone number. Messages are now merged since ios12. They were saved as separate contacts in my phone & now only the last one she used appears in my phone no matter what. Even when I manually enter her phone number or email, it defaults to last used.

This would be helpful if it worked.. My family has separated all personal devices. However, after the update my parents contacts have "merged". This is a very frustrating issue and Apple needs to address this soon. This shouldn't have even happened, never has happened like this. I have two devices (both on iOS 12) and they both receive iMessages. But with my parents, only one device receives a message. Both of their devices are on separate apple id's and separate phone numbers. We've tried signing them out of their icloud and signing back in, but nothings worked. Contacts till merged. The only way to reach them is by calling and that isn't foolproof either.

This is not always the case. My mother has 2 devices under her Apple ID as she should. However she does not always have them BOTH with her. We can only text the device she used last! For example, she texts from iPad while at home, grabs her iPhone & leaves to run errands, texts only go to iPad that has been left at home since she sent messages from iPad last. This is not desirable. Nor is having all messages go to both devices all the time.