There are many ways how to accidentially duplicate files: storing multiple mail attachments, copying or restoring files to a wrong directory. Here is a simple way how to get a lof of disk space back: just run this small utility that will compare file signatures and delete all unused copies.
Don´t miss also the new post in the download section, that will explain how to save 699€ for a groupware calendar as you can sync your phone to Outlook and will also have access to a webspace where php is being enabled.