Prevent duplicates in url list.

Summary:
When not doing that, drag'n'drop-ing an article from the article list
to the browser opened the article twice.

Test Plan: Tested with various RSS feeds dropping to Firefox, no more duplicated tabs

Reviewers: mlaurent, #kde_pim

Reviewed By: mlaurent, #kde_pim

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D24963