Fix for IntegrityError: duplicate key value
Fix for IntegrityError: duplicate key value

Addresses an issue where data from Munki 3 clients could trigger errors like:

IntegrityError: duplicate key value violates unique constraint "server_installedupdate_machine_id_8fddc27d_uniq"
DETAIL:  Key (machine_id, update, update_version)=(46, AcrobatPro9Update, 9.4.4) already exists.

The changed code does extra checks to prevent it from trying to create multiple InstalledUpdate items that share an update_name and version.
sheagcraig/sal,sheagcraig/sal,erikng/sal,erikng/sal,sheagcraig/sal,salopensource/sal,erikng/sal,sheagcraig/sal,salopensource/sal,salopensource/sal,salopensource/sal,erikng/sal