Sony Xperia Z5: multiple numbers wrongfully appear in texts?

[Android Central Question]

Hi all
I have been having an annoying issue with my Z5 for a while. When receiving texts from certain contacts, the message appears to have been sent from multiple senders. Furthermore, even though the numbers that appear are stored as contacts, my phone shows them as numbers rather than names.

Example (text from friend A):

+6612224455; +6615548876
Ipsum delerium, experium vortex and so on. See you whenever...

Instead of:

Friend A
Ipsum delerium, experium vortex and so on. See you whenever...

I often recognize the numbers, so that is not much of an issue. However, I am rarely able to reply directly to the messages (no keypad appears) and if I am, I have experienced that my reply is sent to all numbers in the header!

It seems likely that my issue is caused by some sort of grouping of contacts somewhere, but I have been unable to locate it. It happens mainly, if not exclusively, with contacts using iPhones.

I have tried deleting the conversations with the contacts affected and I have made them do the same, but to no avail

I suspect there is a simple solution that I have overlooked, so any help is appreciated.

Thanks in advance.