My iMessages are sending to multiple devices. Apple ID is not shared between the people receiving these iMessages, so why is it happening?

When i send iMessages to my friend, they send to her dad as well - even though I don't have his phone number or email. They do not share an Apple ID or an email address and are not connected through a home network.

The apple store genius bar told me that it is because of the settings on my friends phone - but they have checked the send/receive settings and it is set to only her phone number.

I asked why it is just my messages going through to her dad, and not messages from all of her contacts, and they told me it is because I am the only person to iMessage her, which simply isn't true, and when I told them this, they were unable to give me another explanation.

Does anyone else have an explaination for why this is happening? I can stop it by turning imessage off but this isn't exactly practical for me as I live somewhere where I rarely get signal, so imessage is my primary use of communication. Any suggestions would be gratefully received!

Many thanks.