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:
{F5669395}

After:
{F5669397}

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