Mac Mail app sending (or at least showing) two sent emails.

I use several Outlook.com email addresses for my personal email. I don't really like Gmail and my ISP, Verizon, switched to AOL for their email, which I don't use. I have noticed for the past few weeks that when I send an email, it shows two copies in the Sent folder. Honestly, I haven't asked anyone if they are getting two copies, but it does show two in the Sent folder and when I log into the Outlook website, I can see two copies there.

Apparently, this is a problem that people are aware of and has been ongoing for some time, but I still haven't found a solution. I tried changing my settings to use the Sent folder on the Mac, and that does work as it shows only one copy in the Sent folder, but changing that setting makes all of my previously sent emails disappear from the folder and I have to change it back to using the Server to see them again.

Are there any solutions to this? I've spent several hours going through threads, but I haven't found anyone that has a definitive solution.

Thanks

 

I have three Outlook accounts. All were set up as IMAP. I deleted all three accounts and set them up again as Exchange and that seems to be working. One nice thing is that I have 3 Macs so with Keychain, the other two updated on their own as well. Thanks for your help.