You subscribed to to the Webhook release event and configured a URL. Later you released a new version of your app and noticed that the endpoint you configured was called 2 times.
We emit one event when the release starts (the action field will be "create") and a second one once the release successfully finishes (with action "update").
If you need to receive only one invocation when the deploy finishes you should build an intermediate app to receive the Webhook triggers, filter the first event (the one with action = create) and call your endpoint only when the second event arrives.
Please note that if you are using release phase tasks you will receive 3 events, a create action and 2 update actions, in this case the final one is the one with action = update, status = succeeded and current = true.