Can you post a screenshot because I'm confused as to what the error is? Are you sure the PM wasn't just sent to both of you or it isn't just an error on the sender's part? He could just have copied and pasted it from another message but without a screenshot people likely can't advise further.
Yup! I agree a screen shot would be necessarily for further inspection if it is a system error or a human error, which I think most Personal message error involve with a typo of a person well because I just did that once, and believe I was shock that a different person PM me of what am I trying to say to my realization that I have the wrong person put to the Receiver of the message!
