Even though you are as careful as possible you can still have a duplicate email, that just takes up space. To solve this problem you have Duplicates Remover for Outlook.
Duplicates Remover for Outlook is a plug-in to add to both MS Outlook and MS Exchange Server that searches and locates all possible duplicate information. It not only looks for emails but also programmed tasks, notes, contacts, etc. It can even detect duplicated information when the names are different.
Duplicates Remover for Outlook doesn't delete all the information found, it just marks it, so you can be the one who decides.