Solution: I have a number of users with large mailboxes, and whenever mailboxes get large Outlook seems to get flaky. On Monday a user with one of these large mailboxes approached me and said that every time he sends an email the recipient gets 2 copies.
After I did a bit of research, here is what I did to resolve the issue:
- I did a full reinstall of Microsoft Office.
- I did a repair of Microsoft Office.
- I read that having a too many items in the sent folder can cause this issue, so I archived items from his sent folder into a separate .pst. I suppose you could also delete unneeded sent items.
- After the users sent folder was reduced Outlook only sends 1 email.