Qt4: correct fix for duplicate items in playlist - service-discovery

Qt4: correct fix for duplicate items in playlist - service-discovery

Hypothesis: The bug stems from a thread concurrency problem. A PLModel::rebuild() happens
just between SD inserting items into the core playlist and Qt playlist receiving item-added callback.
Thus, the items referred to the callback are inserted twice into the Qt playlist.

Solution: check if item already inserted at callback processing.