This is how iMessage works, not contacts, and as a user of the iPhone since its launch in 2007 this is surprisingly unexpected to me. I would expect each separate email/phone number to have its own conversation.
No, that’s only what happens in iMessage or iOS messages. In almost every other context you get one set per number or email, possibly with an option for a combined conversation. That’s really the issue, there’s no indicator in the conversation of which number or email you’re communicating with, or which you’re sending to, and no interface for separating or combining them. It happens non-deterministically at some point after adding multiple to the same contact, sometimes. That’s why I say the UX is poor, it’s unpredictable and uncontrollable even for someone that knows to look.
I am not trying to argue with you. The method of separating or combining emails/numbers to me lies within the contact.
Because imessages can be sent from both emails and phone numbers (e.g. iphone as well as ipad without cellular connection) i’m not sure if there is a way to handle it while also accommodating your preference
Again, this has nothing to do with contacts and everything to do with the interface of the messaging app, as I and the person you responded to said. Also, the person you're replying to literally described a way to handle it while accommodating everyone (offer an indication in the UI of what's happening, and present the option to combine the conversation).
