Having recently adopted Resend and skimmed a bunch of different email APIs, I'm still waiting to find a single provider whose API supports Stripe-style idempotency [1] so that I can guarantee I don't send the same email through their API multiple times. I'd like to confidently avoid accidentally spamming a user if i.e. a background job retries multiple times due to an unrelated error, or merely from failing to receive the API response that an email was sent/created successfully.
Plunk's API does not appear to offer any such feature: https://docs.useplunk.com/api-reference/transactional/send
Unfortunately neither does Resend, Sendgrid, Postmark, etc.
[1]: https://docs.stripe.com/api/idempotent_requests