How can I merge group chats that are breaking and duplicating on iPhone?

I know this was a problem in the past but this has been happening to me and several of my family and co workers. I have two group chats that have this issue. Both contain a mix of iPhone and non iPhone users and ofcourse I think the issue lies with the people who are not iPhone users, but being that as much as I would like them to just get iPhones... they are just simply not going to do that. Randomly on one device or another a chat splits when someone texts something. Nothing is different at all the phone number is the same, they are not responding from their imessage account or anything like that... In fact on my Mac Mini the chat is not broken in two... it has all the texts in one chat, however on my iPhone and Macbook it is showing as two chats. My sister in law is seeing the same thing on her phone but my Brother (all three of us have iPhones) has the chat intact as one chat. WHY! is there a way to merge the chats?

[Re-Titled by Moderator]