'message-read' notification emails sent multiple times

Good evening,
I have activated the option:
Email me when my sent messages are read
Suppose user X sent a message to user Y
when user Y reads the message, an email is sent to user X informing him that user Y has read his message.
The problem is that every time user Y reads the same message, an email is sent 'again' to user X informing him that user Y has read the message.