If you have just a few duplicates, it may be easiest to manually remove them. For emails, this is no problem but for any other Outlook item it is easiest to view the items in a table view, then select the duplicates, then delete them.

To accomplish this, go to View…By Category (or “By” anything really). This gets the view into a mode you can work with. Click either names or subjects (or any shown field) in the column header (in the gray area) to gather similar items together. You can then delete all the duplicates you find.

Online