Nov 21 2018
one of my user reported that she received duplicate emails from the same sender.
she received the 1st email and then replied to the sender. about 20 mins later she received the same email from the sender again.
Nov 22 2018
Apart from checking the message headers and/or running a message trace, there's nothing you can do to troubleshoot this. It's most likely something on the sender's end, either a manual or automatic resubmission of the message.