Discard duplicate results during contact completion

Summary:
Drop duplicate results from contact completion to return
more relevant results. This is still limited by the indexing
side as we are unable to deduplicate easily based on the email
address itself (or merge the results in some clever way).

Reviewers: KDE PIM, dfaure

Reviewed By: dfaure

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D28615