Intercom "Update contacts" destination: 409 conflict race when concurrent invocations create the same new contact

Bug Description

What happens
The Intercom contacts destination (built from the official template) fails with http_4xx when two invocations for the same person run near-simultaneously and the contact doesn't exist in Intercom yet:

Error from intercom api (status 409): {'type': 'error.list', 'errors': [{'code': 'conflict', 'message': 'A contact matching those details already exists with id=...'}]}

Why it happens
The template does search-by-email → create-if-not-found. For a brand-new user, $identify and a $set-carrying event (e.g. sign_up, login, $screen) often arrive within the same few seconds. Both invocations search, both find nothing, both POST a create — the second one gets the 409 and the invocation is marked FAILED. Verified in our project (EU Cloud): within 10s of an $identify, the same users routinely fire multiple $set-carrying events, so any destination matching both Identify and Set person properties races on every new contact.

Expected behavior
The template should treat 409 as "contact exists": parse the id from the error body and retry as an update (PUT /contacts/{id}), i.e. behave as an upsert. Alternatively, use Intercom's upsert semantics where available.

Impact
Every new contact produces failed invocations that pollute error monitoring; data self-heals on the next event but the noise makes real failures hard to spot. Currently working around it by forking the source code, which detaches us from template updates.

Environment

• PostHog EU Cloud
• Destination: Intercom — Update contacts (official template, OAuth connection)
• Reproducible: consistently, on any contact new to Intercom with two matched events within a few seconds

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables or https://eu.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]

Start at the official Intercom contacts destination template and trace the search-by-email, create-if-not-found path described in the report. Reproduce concurrent identify and $set-carrying events for a new contact; done means a create race treats the 409 response as an existing contact, updates it, and no longer marks the invocation failed, with a regression test for the race.