when I "Reply To", receiver sees his content but with my address

Really strange. This is only happening to 1 user right now that I can confirm.
When I hit "Reply To" and send back a message, the receiver does not even see the words that I replied, instead he sees his own earlier message content, but with that content/message being sent from me. In other words, he doesnt even get my message content, and its like I am just sending him back his message.
Has anyone ever seen this before?
Thanks.