Sometimes the supposed duplicates aren't really identical, but differ in some aspects, e.g. (for an e-mail) in the reception timestamp.
When 1-Click Duplicate Delete for Outlook doesn't find any duplicates that are 100% identical, but some similar objects, it automatically suggests to remove them instead. To protect your data, it will only suggest this if the differences are "harmless".
If you need to remove objects that are more different than that, you can use the "Advanced Scan" and defined what you consider a duplicate on the page "How". If you're unsure which properties should be selected, you can find a detailed description in the section Problem solutions..