Removed duplicate preview screenshots

Summary:
Now it checks if the preview screenshot URL is already in the list.
Sometimes it happens that preview1 and preview2 are the same URL.

Test Plan:
Before:

After:

Reviewers: apol, Discover Software Store, davidedmundson, abetts

Reviewed By: apol, Discover Software Store, davidedmundson, abetts

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D10027