Most of the times, that's very easy:
In Outlook visit the folder containing the duplicates and click on the button "1-Click Duplicate Delete". The rest is done automatically.
If you'd like to have more control over the duplicate search, you can use the button "More" and then "Advanced scan". Here you can configure several things with the dialogs "Where", "How" und "What" and confirm the result later on.
If not all duplicates where found, please have a look at the page "How do I remove similar objects?".