shared messages on two phones on same account

We have 2 iphones XR and 12 on the same apple account and have set them up in messages so they both get the same messages and some times they both get them but sometimes on phone will get 30 repies to a text and the one on the next desk only gets 7 of them.

i have tried with wifi on and when off in our office wifi and from a different office and the messages all arrive ok, but external replies from other people not going to both is the issue.

any thoughts

thanks