It`s a good habit to clean up your computer once in a while. Programs you use on a daily base, like Microsoft Outlook tend to get fullier every day. A periodic Microsoft Outlook cleanup, for the inbox, directories aswel as agenda, task list and contact, brings back speed and harddisk space.
For this task I can recommend Automatic Outlook Duplicate Remover.
The steps are easy, but first make a backup of your Outlook PST file(s).
Next download and run Automatic Outlook Duplicate Remover.
I also recommend it to manually cleanup by following this Microsoft post: Reduce the size of Outlook Data Files (.pst and .ost)
The last step is compression, to squeeze out some extra harddisk space. I used KB289987 for this.