Docs: Update Brevo opened event

Brevo has stopped sending the opened
event on first open (along with the
unique_opened event), so change docs
to recommend enabling both.

anymail/webhooks/sendinblue.py

@@ -56,7 +56,7 @@ class SendinBlueTrackingWebhookView(SendinBlueBaseWebhookView):
         "unsubscribe": (EventType.UNSUBSCRIBED, None),
         # shouldn't occur for transactional messages:
         "list_addition": (EventType.SUBSCRIBED, None),
-        "unique_opened": (EventType.OPENED, None),  # you'll *also* receive an "opened"
+        "unique_opened": (EventType.OPENED, None),  # first open; see also opened above
     }
 
     def esp_to_anymail_event(self, esp_event):

docs/esps/brevo.rst

@@ -264,10 +264,13 @@ Be sure to select the checkboxes for all the event types you want to receive. (A
 sure you are in the "Transactional" section of their site; Brevo has a separate set
 of "Campaign" webhooks, which don't apply to messages sent through Anymail.)
 
-If you are interested in tracking opens, note that Brevo has both a "First opening"
-and an "Opened" event type, and will generate both the first time a message is opened.
-Anymail normalizes both of these events to "opened." To avoid double counting, you should
-only enable one of the two.
+If you are interested in tracking opens, note that Brevo has both "First opening"
+and an "Known open" event types. The latter seems to be generated only for the second
+and subsequent opens. Anymail normalizes both types to "opened." To track unique opens
+enable only "First opening," or to track all message opens enable both. (Brevo used to
+deliver both events for the first open, so be sure to check their current behavior
+if duplicate first open events might cause problems for you. You might be able to use
+the event timestamp to de-dupe.)
 
 Brevo will report these Anymail :attr:`~anymail.signals.AnymailTrackingEvent.event_type`\s:
 queued, rejected, bounced, deferred, delivered, opened (see note above), clicked, complained,

tests/test_sendinblue_webhooks.py

@@ -294,8 +294,9 @@ class SendinBlueDeliveryTestCase(WebhookTestCase):
         )
 
     def test_opened_event(self):
-        # SendinBlue delivers unique_opened *and* opened on the first open. To avoid
-        # double-counting, you should only enable one of the two events in SendinBlue.
+        # SendinBlue delivers 'unique_opened' only on the first open, and 'opened'
+        # only on the second or later tracking pixel views. (But they used to deliver
+        # both on the first open.)
         raw_event = {
             "event": "opened",
             "email": "recipient@example.com",
@@ -319,8 +320,7 @@ class SendinBlueDeliveryTestCase(WebhookTestCase):
         self.assertIsNone(event.user_agent)  # SendinBlue doesn't report user agent
 
     def test_unique_opened_event(self):
-        # SendinBlue delivers unique_opened *and* opened on the first open. To avoid
-        # double-counting, you should only enable one of the two events in SendinBlue.
+        # See note in test_opened_event above
         raw_event = {
             "event": "unique_opened",
             "email": "recipient@example.com",