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