I have a friend here with a MacBook Pro early 2016 running Monterey. She has a Hotmail email account and when she sends an email, whilst only one copy is sent, two are put in her Sent Box.
My Googling showed she is not alone but I found solutions either out of date or confusing. Any suggestions would be welcome.
You didn't say which Mail client your friend is using. Some services have an option to include yourself in every message you send's addresses. If that is set, say in Hotmail's preferences, and your Mail client is saving a copy of the sent message (as is usual) you could end up with two copies.
I did not look at the server but both iPhone and Mac on sending immediately show two in Sent Box. I should have checked server but didn't want to risk a password panic!
She is just using Apple's standard Mail apps.
She has gone home now but from problems with a neighbours Hotmail account we reset up and selected the Microsoft Exchange option in the Add Internet Account in Apple preferences/settings. I might try that next time we meet up.
Thanks for the link Tony but I could not work out how to execute on Apple device.
"Under Tools/Account Settings/hotmailaccount/Copies & Folders, uncheck "Place a copy in" since Hotmail/live/outlook accounts automatically copy sent messages to the Sent folder. Same with Gmail."
We did look at Mail Preferences>Accounts>Mailbox behaviours but to no avail.
I use Hotmail on my Windows Laptop and it only leaves one copy in the sent box. I don't use it often so I have not made any adjustments to the program.