I also have this problem. I send a message to my two sons, or two sisters and it often says "delivered to 1". The problem is it doesn't tell me which one. Then I have to send another text asking who got the first one, etc. It could take all day!
We want to make sure all your messages are delivered. Let's get a few details. What model phone are you using? Are you using the messaging app that came with the phone?