Fix list_user_folders() returning duplicates

Summary:
The result could contain the same folder twice, which later in the code
would lead to doing a search for an event in the same folder twice.

Reviewers: PyKolab Developers, mollekopf

Reviewed By: PyKolab Developers, mollekopf

Subscribers: PyKolab Developers

Differential Revision: https://git.kolab.org/D4493