Avoid processing a webhook event twice when having two endpoints pointing to the same URL in the Stripe dashboard

[a-danae]

Describe the bug
We're processing webhook events twice when two endpoints are configured for the same URL in the Stripe dashboard.

To Reproduce

1. Ensure the Legacy checkout experience is disabled

2. Ensure your test site is publicly accessible so it can receive webhooks

3. Go to your list of configured webhooks at https://dashboard.stripe.com/webhooks

4. Check out the table under "Hosted endpoints"

5. Create two endpoints pointing to the same URL. Like this
   

6. As a shopper, add a subscription to your cart

7. Place the order

8. As the merchant, go to the Edit order page for the latest order

9. Notice there are two order notes with the "Charge complete (...)" message
   

Expected behavior
A single webhook event must only be processed once when there are multiple webhooks configured for the same endpoint.

Additional context

• Try replicating the issue without having breakpoints. When we add breakpoints, the pause in the flow allows the order to be updated before the second webhook is processed.
• This doesn't seem to be the root of the duplicated notes issue - A user having a duplicated notes issue confirmed they have a single endpoint in their webhook configuration. But it's worth to have a more robust approach when facing a potential misconfiguration

There are at least two flows that could be improved:

• When no webhook secret is configured in the store, we must bail out and not process the webhook.

  • Not having a webhook secret is possible in Stripe < 8.6.0 where the webhook secret can be added manually.
  • When no secret is set, we must fail the validation instead of using a fallback based on the user agent.

• When the webhook secret is configured in the store, the invalid webhook is still processed.

  • We're in fact failing the validation when the webhook secret validation fails, but the webhook is still processed for some reason.

[a-danae]

This should have been fixed by #3331