Webhooks `Unique events only`

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/31488

It would be nice to be able to create a webhook that has Issue events, for example, that only fire for unique issue reference. Or in other words, it never fires an event referencing any issue more than once.

For example, if I'm looking to capture all ~Frontend labelled issues, I don't want that issue to keep firing webhooks after its already been fired once.