I think some email systems now allow senders to send their message and then, after it is sent, edit it again if they discover a mistake, and resend the mail. I suspect if you are on the same email system, it only shows the latest edit, but not when you are using the Thunderbird client. The result when I am reading my email in Thunderbird is many copies of almost the same message, one for each time the careless sender re-edited their message. Often these messages contain at the top, a message " [Edited Message Follows] ". Perhaps Thunderbird could use this information to create a single edited message, so it doesn't cause me to read the same email twice with minor changes, and it would act like the original email system the sender is using.