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.
Thanks,
Rob