I can't really find an answer to this. It's happened to me before but the only way I get confirmation of this happening is when my recipients mention it to me:
I send a mass text to say, 2 people. One person hits reply to reply to my message and that reply goes to the other person I sent the message to as well. prompting them to ask "who is this?" to my other recipient.
I am going to experiment with this tonight when I go to a friend's house because there will be 5 of us with iPhones there. Haha!
A semi-official response would be nice too though.