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I still have the issue in beta 9. Understand that I did not have this issue at all on IOS 11, the only thing that has changed on my account was installing IOS 12 on my device only. We all have our own phone number and this should work. It is a bug. we should not have to change our accounts in order to correct and Apple problem. I have had 6 Iphones on the same Apple ID for 5+ years with out an issue. All 5 of the other Iphones on my ID do not have this issue and they are on IOS 11.
 
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Just wanted to throw this in there. Having the same problem on my iphone 6s since one of the earlier betas. I have reported this in feeback as well.

The only work around I've found is to text the right person from my watch or ipad (still running ios 10) which will force the conversation back to the person that I meant to text. On both those devices the threads are separate.

Glad to know I wasn't the only one with this issue, been very annoying.
 
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Having this problem as well, and it's happening in the Mojave betas now, too. I wonder if it's due to iCloud Messages (my setting, not theirs)?
 
Having this problem as well, and it's happening in the Mojave betas now, too. I wonder if it's due to iCloud Messages (my setting, not theirs)?

I turned off iMessage and it was still happening. Never happened on IOS 11.
 
I think it goes back to the change in Messages that threads with the same Message ID will end up getting merged. I am starting the process of not using the same Apple ID across devices (and for other family members too) to have them setup and prepped before iOS 12 launches and causes things to go haywire.
 
So I disconnected my sons email and got him his own ID. When sending him a message it merges with my wife’s thread, who is also on her own iMessage ID.

So either there is still a glitch or I have to go full on iCloud and family sharing switches on their devices.
 
I signed out of the icloud account on all 3 phones and it is still combining all 3 people into 1 thread. Apple is gonna have a lot of angry people if they let this get out into the wild.
 
This is now happening on my iPhone X after the update. I try to text my son but it shows text history from my daughter. This is a mess! Any thoughts?
 
This is now happening on my iPhone X after the update. I try to text my son but it shows text history from my daughter. This is a mess! Any thoughts?

The only way I fixed it was to sign out of messages on the shared iCloud account, create a separate Apple ID and then sign in to messages/FaceTime with the new ID.

Then I had to delete my message history where it was merged (after saving any images I wanted to keep) and then it’s worked since.
 
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