Other people's conversations

I have just set up what's app on another phone due to mine breaking, soon as it set up I got a few messages through of mine from group chats but then has two messages on their which I haven't sent nor do I know who they are and it has two sent items where the receiptent hasn't read them but they have delivered. Is this simply because the other person hasn't wiped their phone or will it of sent to them when I set mine up, just worried of randomers having my number.