pghkp: report duplicates when inserting
pghkp: report duplicates when inserting

Reviewed-on: https://code.launchpad.net/~pjdc/canonical-is-hockeypuck/+git/canonical-is-hockeypuck/+merge/368321
Reviewed-by: Joel Sing <7ca239bdbcd314d2bf90bce5a71ad53794699165@canonical.com>

cmars/hockeypuck,cmars/hockeypuck