is it possible for messages sent to 1 person be shared between 2 different iPhones

I get that I can send a shared message to my son and daughter in law, but recently by accident when they were staying with me I noticed a message I sent my son and just my son showed up on my daughter in laws phone as well, they both have iPhones. If so is it possible for me to make sure that when I’m sending him an SMS that it’s just going to him