Support for deduplication keys

I'd love to see dedup key support added.

The idea being that if multiple souces send the same notification, only the first source to call the API with a configurable time window will trigger a notification because all source share the same deduplication key.

For example, when multiple redundant monitoring systems are the souces of an notification for the same event (like a host goes down), only the first source to report the event triggers a notification, and all subsequent sources sharing the same dedup key for that event will be dropped.