Fixed crash when unloading NPC (bug #7517)

Follow-up to 6e0da189207134c8d57ecca48f397bc592dcc1ad
Bug report:
http://hercules.ws/board/tracker/issue-7517-yet-another-map-server-cr/

Duplicate events are now skipped instead of overwriting existing ones.
Merge pull request #58 from piotrhalaczkiewicz/master

Fixed crash when unloading NPC (bug #7517)