We have two Windows 10 computers, both up to date with all Windows updates. Both have Office 365 and both use the same email address, set up as imap. Email extension is verizon.net.
So if we send an email from one computer, it does put the email in the sent folder for the originating computer AND the other computer. However, it OFTEN puts TWO copies of the same sent email in the sent folder on both computers! This doesn't happen all the time. I've checked the box "Do not save the copies of sent items” in settings - on both computers, but that hasn't changed anything. It is still doubling the sent email - on occasion! I've played around with both of them, turning off one Outlook program, and it does appear that if one computer's Outlook is NOT on, the doubling doesn't occur!